Followup bd3906ce64
Converts the new endpoints made in the original PR into
service classes, using AI to drive implementation based
on the service authoring skill:
* CustomEmoji::Export
* CustomEmoji::PreviewImport
* CustomEmoji::ConfirmImport
In addition, some supporting action, lib, and serializer
classes have been added. This fixed a couple of
bugs as well:
* Import names now go through `Emoji.sanitize_emoji_name` like
single-emoji create
* No-group imports no longer persist a literal "default" string
* Two 500s (suspicious zip entries, malformed CSV) became proper 422s
* Duplicate CSV names are flagged at preview instead of exploding the
confirm transaction
*` Emoji.clear_cache `moved outside the transaction