* Custom emoji editor: implement backend
This reuses the logo upload code
* Implement emoji edit admin interface
Again reuse base64 logic from the logo upload
* Allow toggling between uploaded and default emojis
* Add route that always serves uploaded emojis
This is needed for the admin emoji interface,
as otherwise the emojis will 404 if custom emojis are disabled
* Fix linter warnings
* Remove custom/uploaded emoji logic
* Reset timer after emoji deletion
* Setup: copy built-in emojis to emoji directory
