Add tag deletion/addition filters for conflict resolution page

[caseyhans]

Adds additional filtering to conflict resolution page.

• Added deleted_tags field to UserReferenceTag model
• Candidate Tag Additions: filters by references with users seeking to add a specific tag (tag is present for an unresolved UserReferenceTag, but not on reference.tags)
• Candidate Tag Deletions: filters by references with users seeking to remove a specific tag (tag is in user_tag.deleted_tags)

• Show tag deletions on Tag Status page

• Show text for empty UserReferenceTag case
  

• Change empty conflict resolution list message from success ("Conflict resolution complete") to "No conflicts found"
  

[shapiromatron]

Really nice job. This was a fun read.

Made some minor changes; if you're ok, feel free to merge!

• ef5786f reuse tag queryset for each field instead of multiple queries
• 0cda1b0 use == instead of is for numerical comparison
• a07c65f use alert template tag; add icon template tag

[shapiromatron]

this was a joy to read; nice job.

[shapiromatron]

These methods look good, but they could be slow. I don't think we need to change anything now.

We may need to monitor performance with some real data after its in the wild for a while to determine if we need to optimize. I'm thinking specifically of a case where we include descendants, and the parent has many children. I wonder if there's a way to see to see if a reference has any of these tags in a single query.

I think you could do it it with the ArrayField using overlap, but that's only for the deletions, not the additions.