Show change tags on diffs

This adds two very simple queries for every diff page view.

It could be made to only add one (loading tags for both revisions at
once), but it would be a little ugly.

It could even be made to add zero, but this would require either
rewriting and duplicating a lot of code here and constructing
Revisions by hand or making the Revision itself know about its tags.

Bug: 25824
Bug: 49602
Change-Id: Ic2ae58c703db7ceee5de4b320229d8c93810a73b
Merge "Show change tags on diffs"