[feature] Allow admins to upload / delete custom emojis via admin panel

[tsmethurst]

The admin section of the settings panel needs a subsection for emojis.

Here, admins should be able to:

• View custom emojis present on the instance
• (GET /api/v1/admin/custom_emojis[?filter=local], returns an array of emojis like from /api/v1/custom_emojis but including domain, date uploaded, last updated, id, etc).
• Upload a new custom emoji, with a shortcode and category (POST /api/v1/admin/custom_emojis) -- ideally, admins can drag and drop an emoji file into a box and see how it'll actually look when rendered on the instance, but this is probably FIDDLY so basic implementation of selecting a file + uploading it is fine to start with.
• Delete a custom emoji, either local or remote (DELETE /api/v1/admin/custom_emojis/{id})
• Use PUT or PATCH? to /api/v1/admin/custom_emojis/{id}:
  • Modify a local custom emoji -- set a new image for it, put it in a different category, etc
  • Disable/suspend a remote custom emoji
  • Copy a remote emoji to local

[tsmethurst]

Gonna work on this one now: #821

And then work on the PATCH endpoint mentioned above :)

[xplosionmind]

When visiting https://gts.superseriousbusiness.org/settings/admin/custom-emoji I get this.

An error occured, please report this on the [GoToSocial issue tracker](https://github.com/superseriousbusiness/gotosocial/issues) or [Matrix support room](https://matrix.to/#/#gotosocial-help:superseriousbusiness.org).
Include the details below:

TypeError: t is undefined

[112]</module.exports/r<@https://gts.superseriousbusiness.org/assets/dist/settings.js:2831:1567
useMemo@https://gts.superseriousbusiness.org/assets/dist/settings.js:2268:70076
[75]</exports.useMemo@https://gts.superseriousbusiness.org/assets/dist/settings.js:1785:6231
[112]</module.exports@https://gts.superseriousbusiness.org/assets/dist/settings.js:2831:1548
Xh@https://gts.superseriousbusiness.org/assets/dist/settings.js:2268:63826
Wk@https://gts.superseriousbusiness.org/assets/dist/settings.js:2268:120234
Vk@https://gts.superseriousbusiness.org/assets/dist/settings.js:2268:111701
Uk@https://gts.superseriousbusiness.org/assets/dist/settings.js:2268:111632
Jk@https://gts.superseriousbusiness.org/assets/dist/settings.js:2268:111499
Ok@https://gts.superseriousbusiness.org/assets/dist/settings.js:2268:107940
Fk@https://gts.superseriousbusiness.org/assets/dist/settings.js:2268:108744
jg@https://gts.superseriousbusiness.org/assets/dist/settings.js:2268:47683
[38]</Ek/<@https://gts.superseriousbusiness.org/assets/dist/settings.js:2268:106280