Emoji picker for PieFed reactions and markdown editor

[jwr1]

In this PR:

• PieFed emoji reactions are displayed above the post/comment action row (even in the feed).

• While signed in, clicking on a present emoji reaction will select the same emoji for you.

• Custom server specific emoji reactions are displayed.

• An emoji picker (used for PieFed reactions and MD editor), with support for searching emojis, and emoji groups with buttons to scroll to the section. Emoji data is pulled in from emojibase on app build (with build_runner). Emoji searching uses a Trie data structure for super efficient string prefix lookup (based on emoji labels, tags, and emoticons).

In another PR:

• Custom emoji picker groups, including quick access emojis and PieFed server specific emojis.
• If the screen is too narrow for the current emoji picker, split emoji groups across two lines and use that as the picker width.

[olorin99]

Just realised that the emoji picker is available on mbin when it should be piefed only. Also the edge of the picker can be cut off on mobile making difficult to get to the last couple of categories. So maybe when on mobile show the picker in a bottom modal or something that is constrained to the screen. The floating menu is good for desktop though.

[jwr1]

I think it would be a good idea to create an abstract menu system. One that displays full screen bottom sheet modals on small screens, but displays floating menus on large screens. I won't do that in this pr though.