Post-refactoring and improving of the new batch tags update flow

[MareStare]

There are several changes packed in this single PR

Zero changes edge case

Fixed this edge case when the user doesn't select any tags to revert at all and clicks on the "Revert selected" button:

(this bug should active be in prod too).

Eliminate redundant preloads

I removed the redundant :tag and :tag_change (reverse) preloads for the initial TagChange select. The inside Images.batch_update doesn't need the full TagChange.Tag struct, it can simply work with tag_ids lists.

More info in the flash

Also added the number of actually updated tags to the result of the revert:

TODO

I'll try to add some auto tests for this code

[Meow]

You should've asked me why there was mass_revert_tags... I intend to implement per-tag reverts as a mod tool in the future, so that mods can select individual tags to be reverted as opposed to whole batches. Hence why this whole structure and tag_changes relation preloads on TagChanges.Tag.

[Meow]

Split refactoring / logic fixes, and unit tests, into two separate PRs, this PR is becoming way too large. Maybe even more PRs to be honest, your initial fixes were small and easily mergeable, but this has gotten really, really huge now.