* 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.
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