* Add a `rename A -> B` command for bulk update requests.
* Change mass updates to only retag the posts, not to move saved
searches or blacklists.
A tag rename does the same thing an alias does, except it doesn't
create a permanent alias. More precisely, a tag rename:
* Moves the wiki.
* Moves the artist entry.
* Moves saved searches.
* Moves blacklists.
* Merges the wikis, if both tags have wiki pages.
* Merges the artist entries, if both tags have artist pages.
* Fixes links in wiki pages to point to the new tag.
* Retags the posts.
Remove the post update count estimate from BUR show pages. This was
complex, slow, and usually inaccurate since it assumed that requests in
a BUR had no overlap with each other, which usually wasn't the case.
Also fixes a bug where mod actions weren't logged on mass updates.
Creating the mod action silently failed because it was called when
CurrentUser wasn' set.
