MBS-960: Allow mass-deleting and mass-changing user tags

[reosarevok]

Implement MBS-960

See commit messages for details.

Tested manually with several different combinations of situations (changing tags into some that do not exist and some that do, deleting upvotes, deleting downvotes).

[reosarevok]

Converting to draft for now: the best way to do this is probably to first make sure we have triggers to keep raw and grouped tag tables in sync, rather than doing it all via code. See also stuff like MBS-5359 about why this is a mess.

[reosarevok]

The triggers are now present since the 2022 Q2 schema change, so this should work fine now.

[reosarevok]

I wrote some tests for this too, but do check in case you feel it needs more test cases :)