Apply and enforce file-level license headers

Use `mattermost-govet` and `eslint`'s `header/header` to ensure each
file begins with the appropriate header, then fix all instances.

```
// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
// See LICENSE.txt for license information.
```

Note that this renames `LICENSE` to `LICENSE.txt` purely to align on the
copyright messaging across repos.

Makefile

@@ -160,6 +160,7 @@ install-go-tools:
 	@echo Installing go tools
 	$(GO) install github.com/golangci/golangci-lint/cmd/[email protected]
 	$(GO) install gotest.tools/[email protected]
+	$(GO) install github.com/mattermost/mattermost-govet/v2@51f518d9d62ea9f3bbcb172a1e52449f13940195
 
 ## Runs eslint and golangci-lint
 .PHONY: check-style
@@ -178,6 +179,7 @@ ifneq ($(HAS_SERVER),)
 	@echo Running golangci-lint
 	$(GO) vet ./...
 	$(GOBIN)/golangci-lint run ./...
+	$(GO) vet -vettool=$(GOBIN)/mattermost-govet -license ./...
 endif
 
 ## Builds the server, if it exists, for all supported architectures, unless MM_SERVICESETTINGS_ENABLEDEVELOPER is set

build/manifest/main.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package main
 
 import (

build/pluginctl/logs.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package main
 
 import (

build/pluginctl/logs_test.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package main
 
 import (

build/pluginctl/main.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 // main handles deployment of the plugin to a development server using the Client4 API.
 package main
 

client/client.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package client
 
 import (

client/client_test.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package client
 
 import (

plugin.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package root
 
 import (

server/main.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package main
 
 import (

server/plugin/api.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/plugin/api_test.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/plugin/cluster.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/plugin/command.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/plugin/command_test.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/plugin/configuration.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/plugin/configuration_test.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/plugin/flows.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/plugin/graphql/client.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package graphql
 
 import (

server/plugin/graphql/lhs_query.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package graphql
 
 import (

server/plugin/graphql/lhs_request.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package graphql
 
 import (

server/plugin/mm_34646_token_refresh.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/plugin/oauth.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/plugin/permalinks.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/plugin/permalinks_test.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/plugin/plugin.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/plugin/subscriptions.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/plugin/subscriptions_test.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/plugin/support_packet.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/plugin/telemetry.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/plugin/template.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/plugin/template_test.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/plugin/utils.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/plugin/utils_test.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/plugin/webhook.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package plugin
 
 import (

server/testutils/http.go

@@ -1,3 +1,6 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
 package testutils
 
 import (

webapp/.eslintignore

@@ -0,0 +1 @@
+src/manifest.ts

webapp/.eslintrc.json

@@ -11,7 +11,8 @@
   "plugins": [
     "react",
     "@typescript-eslint",
-    "import"
+    "import",
+    "header"
   ],
   "env": {
     "browser": true,
@@ -665,6 +666,12 @@
           {
             "extensions": [".jsx", ".tsx"]
           }
+        ],
+        "header/header": [
+            2,
+            "line",
+            " Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.\n See LICENSE.txt for license information.",
+            2
         ]
       }
     }