Custom emojis support

[hatkidchan]

It'd be nice to show custom emojis when you compose posts, like Mastodon web does:
(yes, I know there is "Insert emoji" button with "Custom" tab in it, it's just not as convenient to use as Mastodon default one)

Also custom emojis don't show up in "Notifications" tab:

[alicerunsonfedora]

We'll keep this in mind for Starlight and Luna, provided that we can implement it in the tools we use to build those.