Scala 2.13 Scalafix: ExplicitResultTypes for implicit definitions

This is a precursor to:

* #536

Scala 2.13.11 added a warning that implicit definitions [should](
https://nrinaudo.github.io/scala-best-practices/tricky_behaviours/type_implicits.html) have an explicit type (because implicit resolution is already complicated enough for the compiler, and things like file-order can actually make a difference to whether implicit scopes are correctly searched: scala/bug#8697 (comment)):

* scala/scala#10083

The warning looks like this:

```
[error] ~/code/presence-indicator/app/autoscaling/Notification.scala:7:16: Implicit definition should have explicit type (inferred play.api.libs.json.Reads[autoscaling.Notification]) [quickfixable]
[error]   implicit val jsonReads = Json.reads[Notification]
[error]                ^
```

...this prepares us for Scala 3, where the explicit type is _required_.

More widely, beyond implicit definitions, in **library** code, the best practice is to always add an explicit type to all your **public** members, even when you're happy with what's being inferred - otherwise you can unintentionally break binary-compatibility just by changing the _implementation_ of a field:

* https://docs.scala-lang.org/scala3/guides/migration/incompat-type-inference.html
* https://nrinaudo.github.io/scala-best-practices/binary_compat/explicit_type_annotations.html
* https://scalacenter.github.io/scalafix/docs/rules/ExplicitResultTypes.html

## Automatically fixing this code issue

Scalafix actually does a better job than `-quickfix` for this particular task, because it adds imports if it needs to, so that you end up with this in your code:

```
implicit val jsonReads Reads[Notification] = Json.reads[Notification]
```

...rather than something like:

```
implicit val jsonReads Reads[com.gu.blah.foo.bar.Notification] = Json.reads[Notification]
```

### Fixing while still on Scala 2.12 - use Scalafix

In this commit, we're only trying to fix the implicit definitions, so I've added this in a new `.scalafix.conf` config file:

```
ExplicitResultTypes.onlyImplicits = true
```

The Scalafix rule needs to be run while the project is still on Scala 2.12, not Scala 2.13
(otherwise sbt will say: "Error downloading ch.epfl.scala:sbt-scalafix;sbtVersion=1.0;scalaVersion=2.13:0.13.0").

Once the Scalafix plugin is made available to sbt (by adding `addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.13.0")`
to either `project/plugins.sbt` or `~/.sbt/1.0/plugins.sbt`), you can run these commands on the sbt prompt to automatically generate the changes in this PR:

```
scalafixEnable
scalafixAll ExplicitResultTypes
```

app/model/CampaignInformation.scala

@@ -18,7 +18,7 @@ case class CampaignInformation(campaignType: String) {
 
 object CampaignInformation {
 
-  implicit val trackingFormat = Jsonx.formatCaseClass[CampaignInformation]
+  implicit val trackingFormat: OFormat[CampaignInformation] = Jsonx.formatCaseClass[CampaignInformation]
 
   def apply(thriftCampaignInformation: ThriftCampaignInformation): CampaignInformation =
     CampaignInformation(

app/model/ClientConfig.scala

@@ -2,6 +2,7 @@ package model
 
 import ai.x.play.json.Encoders.encoder
 import ai.x.play.json.Jsonx
+import play.api.libs.json.OFormat
 
 case class ClientConfig(username: String,
                         capiUrl: String,
@@ -15,5 +16,5 @@ case class ClientConfig(username: String,
                        )
 
 object ClientConfig {
-  implicit val clientConfigFormat = Jsonx.formatCaseClass[ClientConfig]
+  implicit val clientConfigFormat: OFormat[ClientConfig] = Jsonx.formatCaseClass[ClientConfig]
 }

app/model/ContributorInformation.scala

@@ -36,7 +36,7 @@ case class ContributorInformation(
 
 object ContributorInformation {
 
-  implicit val contributorInformationFormat = Jsonx.formatCaseClass[ContributorInformation]
+  implicit val contributorInformationFormat: OFormat[ContributorInformation] = Jsonx.formatCaseClass[ContributorInformation]
 
   def apply(thriftContributorInformation: ThriftContributorInformation): ContributorInformation =
     ContributorInformation(

app/model/EditionalisedPage.scala

@@ -3,14 +3,15 @@ package model
 import ai.x.play.json.Jsonx
 import ai.x.play.json.Encoders.encoder
 import com.gu.tagmanagement.{EditionalisedPage => ThriftEditionalisedPage}
+import play.api.libs.json.OFormat
 
 case class EditionalisedPage(path: String, pageId: Long) {
   def asThift = ThriftEditionalisedPage(path, pageId)
 }
 
 object EditionalisedPage {
 
-  implicit val editionalisedPageFormat = Jsonx.formatCaseClass[EditionalisedPage]
+  implicit val editionalisedPageFormat: OFormat[EditionalisedPage] = Jsonx.formatCaseClass[EditionalisedPage]
 
   def apply(thriftEditionalisedPage: ThriftEditionalisedPage): EditionalisedPage =
     EditionalisedPage(thriftEditionalisedPage.path, thriftEditionalisedPage.pageId)

app/model/ExternalReferenceType.scala

@@ -10,6 +10,7 @@ import play.api.libs.json.{Format, JsPath, JsValue, Json}
 import com.gu.tagmanagement.{Section => ThriftSection}
 
 import scala.util.control.NonFatal
+import play.api.libs.json.OFormat
 
 case class ExternalReferenceType (
                     typeName: String,
@@ -20,7 +21,7 @@ case class ExternalReferenceType (
 
 object ExternalReferenceType extends Logging {
 
-  implicit val sectionFormat = Jsonx.formatCaseClass[ExternalReferenceType]
+  implicit val sectionFormat: OFormat[ExternalReferenceType] = Jsonx.formatCaseClass[ExternalReferenceType]
 
   def fromItem(item: Item) = try{
     Json.parse(item.toJSON).as[ExternalReferenceType]

app/model/HyperMedia.scala

@@ -4,7 +4,7 @@ import services.Config.conf
 
 
 case class LinkEntity(rel: String, href: String)
-object LinkEntity { implicit val jsonWrites = Json.writes[LinkEntity] }
+object LinkEntity { implicit val jsonWrites: OWrites[LinkEntity] = Json.writes[LinkEntity] }
 
 case class EmbeddedEntity[T](uri: String,
                              data: Option[T] = None,
@@ -59,7 +59,7 @@ object CollectionResponse {
 case class EmptyResponse(links: Option[List[LinkEntity]] = None) {
   def addLink(rel: String, href: String) = copy(links = Some(LinkEntity(rel, href) :: (links getOrElse Nil)))
 }
-object EmptyResponse { implicit val jsonWrites = Json.writes[EmptyResponse] }
+object EmptyResponse { implicit val jsonWrites: OWrites[EmptyResponse] = Json.writes[EmptyResponse] }
 
 object HyperMediaHelpers {
   def tagUri(id: Long): String = s"https://tagmanager.${conf.pandaDomain}/hyper/tags/${id}"

app/model/ImageAsset.scala

@@ -3,6 +3,7 @@ package model
 import ai.x.play.json.Jsonx
 import ai.x.play.json.Encoders.encoder
 import com.gu.tagmanagement.{ImageAsset => ThriftImageAsset}
+import play.api.libs.json.OFormat
 
 case class ImageAsset(imageUrl: String, width: Long, height: Long, mimeType: String) {
   def asThrift = ThriftImageAsset(imageUrl, width, height, mimeType)
@@ -15,7 +16,7 @@ case class ImageAsset(imageUrl: String, width: Long, height: Long, mimeType: Str
 }
 
 object ImageAsset {
-  implicit val imageAssetFormat = Jsonx.formatCaseClass[ImageAsset]
+  implicit val imageAssetFormat: OFormat[ImageAsset] = Jsonx.formatCaseClass[ImageAsset]
 
   def apply(thriftImageAsset: ThriftImageAsset): ImageAsset = ImageAsset(
     imageUrl = thriftImageAsset.imageUrl,

app/model/PaidContentInformation.scala

@@ -3,6 +3,7 @@ package model
 import ai.x.play.json.Jsonx
 import ai.x.play.json.Encoders.encoder
 import com.gu.tagmanagement.{PaidContentInformation => ThriftPaidContentInformation}
+import play.api.libs.json.OFormat
 
 case class PaidContentInformation(paidContentType: String, campaignColour: Option[String] = None) {
 
@@ -19,7 +20,7 @@ case class PaidContentInformation(paidContentType: String, campaignColour: Optio
 
 object PaidContentInformation {
 
-  implicit val paidContentInformationFormat = Jsonx.formatCaseClass[PaidContentInformation]
+  implicit val paidContentInformationFormat: OFormat[PaidContentInformation] = Jsonx.formatCaseClass[PaidContentInformation]
 
   def apply(thriftPaidContentInformation: ThriftPaidContentInformation): PaidContentInformation =
     PaidContentInformation(

app/model/PublicationInformation.scala

@@ -24,7 +24,7 @@ case class PublicationInformation(
 
 object PublicationInformation {
 
-  implicit val publicationInformationFormat = Jsonx.formatCaseClassUseDefaults[PublicationInformation]
+  implicit val publicationInformationFormat: OFormat[PublicationInformation] = Jsonx.formatCaseClassUseDefaults[PublicationInformation]
 
   def apply(thriftPublicationInformation: ThriftPublicationInformation): PublicationInformation =
     PublicationInformation(

app/model/Reference.scala

@@ -25,7 +25,7 @@ case class Reference(`type`: String, value: String, capiType: Option[String]) {
 
 object Reference {
 
-  implicit val referenceFormat = Jsonx.formatCaseClass[Reference]
+  implicit val referenceFormat: OFormat[Reference] = Jsonx.formatCaseClass[Reference]
 
   def apply(thriftReference: ThriftReference): Reference = Reference(thriftReference.`type`, thriftReference.value, thriftReference.capiType)
 

app/model/Sponsorship.scala

@@ -99,7 +99,7 @@ case class DenormalisedSponsorship (
 
 object DenormalisedSponsorship {
 
-  implicit val denormalisedSponsorshipFormat = Jsonx.formatCaseClass[DenormalisedSponsorship]
+  implicit val denormalisedSponsorshipFormat: OFormat[DenormalisedSponsorship] = Jsonx.formatCaseClass[DenormalisedSponsorship]
 
   def apply(s: Sponsorship): DenormalisedSponsorship = {
     new DenormalisedSponsorship(

app/model/Tag.scala

@@ -248,7 +248,7 @@ case class DenormalisedTag (
 
 object DenormalisedTag{
 
-  implicit val tagFormat = Jsonx.formatCaseClassUseDefaults[DenormalisedTag]
+  implicit val tagFormat: OFormat[DenormalisedTag] = Jsonx.formatCaseClassUseDefaults[DenormalisedTag]
 
   def apply(t: Tag): DenormalisedTag = DenormalisedTag(
     id = t.id,

app/model/TagSearchResult.scala

@@ -2,9 +2,10 @@ package model
 
 import ai.x.play.json.Jsonx
 import ai.x.play.json.Encoders.encoder
+import play.api.libs.json.OFormat
 
 case class TagSearchResult(tags: List[Tag], count: Int)
 
 object TagSearchResult {
-  implicit val tagSearchResultFormat = Jsonx.formatCaseClassUseDefaults[TagSearchResult]
+  implicit val tagSearchResultFormat: OFormat[TagSearchResult] = Jsonx.formatCaseClassUseDefaults[TagSearchResult]
 }

app/model/TrackingInformation.scala

@@ -18,7 +18,7 @@ case class TrackingInformation(trackingType: String) {
 
 object TrackingInformation {
 
-  implicit val trackingFormat = Jsonx.formatCaseClass[TrackingInformation]
+  implicit val trackingFormat: OFormat[TrackingInformation] = Jsonx.formatCaseClass[TrackingInformation]
 
   def apply(thriftTrackingInformation: ThriftTrackingInformation): TrackingInformation =
     TrackingInformation(

app/model/command/BatchTagCommand.scala

@@ -7,6 +7,7 @@ import ai.x.play.json.Encoders.encoder
 import repositories._
 
 import scala.concurrent.{Future, ExecutionContext}
+import play.api.libs.json.OFormat
 
 case class BatchTagCommand(contentIds: List[String], toAddToTop: Option[Long], toAddToBottom: List[Long], toRemove: List[Long]) extends Command {
   type T = Unit
@@ -37,5 +38,5 @@ case class BatchTagCommand(contentIds: List[String], toAddToTop: Option[Long], t
 }
 
 object BatchTagCommand {
-  implicit val batchTagCommandFormat = Jsonx.formatCaseClassUseDefaults[BatchTagCommand]
+  implicit val batchTagCommandFormat: OFormat[BatchTagCommand] = Jsonx.formatCaseClassUseDefaults[BatchTagCommand]
 }

app/model/command/CreateTagCommand.scala

@@ -17,6 +17,7 @@ import play.api.Logging
 import services.KinesisStreams
 
 import scala.concurrent.{Future, ExecutionContext}
+import play.api.libs.json.OFormat
 
 case class InlinePaidContentSponsorshipCommand(
                          validFrom: Option[DateTime],
@@ -53,7 +54,7 @@ case class InlinePaidContentSponsorshipCommand(
 }
 
 object InlinePaidContentSponsorshipCommand {
-  implicit val inlinePaidContentSponsorshipFormat = Jsonx.formatCaseClassUseDefaults[InlinePaidContentSponsorshipCommand]
+  implicit val inlinePaidContentSponsorshipFormat: OFormat[InlinePaidContentSponsorshipCommand] = Jsonx.formatCaseClassUseDefaults[InlinePaidContentSponsorshipCommand]
 }
 
 case class CreateTagCommand(
@@ -207,5 +208,5 @@ case class CreateTagCommand(
 
 object CreateTagCommand {
 
-  implicit val createTagCommandFormat = Jsonx.formatCaseClassUseDefaults[CreateTagCommand]
+  implicit val createTagCommandFormat: OFormat[CreateTagCommand] = Jsonx.formatCaseClassUseDefaults[CreateTagCommand]
 }

app/model/forms/SpreadSheet.scala

@@ -34,6 +34,6 @@ object GetSpreadSheet {
     }
   }
 
-  implicit val filterFormat = Json.reads[SpreadSheetFilter]
-  implicit val format = Json.reads[GetSpreadSheet]
+  implicit val filterFormat: Reads[SpreadSheetFilter] = Json.reads[SpreadSheetFilter]
+  implicit val format: Reads[GetSpreadSheet] = Json.reads[GetSpreadSheet]
 }

app/model/image.scala

@@ -19,7 +19,7 @@ case class Image(imageId: String, assets: List[ImageAsset]) {
 }
 
 object Image {
-  implicit val imageFormat = Jsonx.formatCaseClass[Image]
+  implicit val imageFormat: OFormat[Image] = Jsonx.formatCaseClass[Image]
 
   def apply(thriftImage: ThriftImage): Image = Image(
     imageId = thriftImage.imageId,

app/model/jobs/Step.scala

@@ -172,7 +172,7 @@ object Step extends Logging {
     }
   }
 
-  implicit val stepFormat = Format(stepReads, stepWrites)
+  implicit val stepFormat: Format[Step] = Format(stepReads, stepWrites)
 }
 
 // Step status is required so we know which steps require rollback

app/modules/sponsorshiplifecycle/SponsorshipLifecycleModule.scala

@@ -86,7 +86,7 @@ class SponsorshipLauncher(implicit ec: ExecutionContext) extends AbstractSchedul
 }
 
 class SponsorshipExpirer(implicit ec: ExecutionContext) extends AbstractScheduledService with Logging {
-  implicit val username = Some("Sponsorship expirer")
+  implicit val username: Some[String] = Some("Sponsorship expirer")
 
   override def runOneIteration(): Unit = try {