Replaces #1980, #2127, and #2221. All previous commits have been merged and rebased. I've also fixed a couple of formatting issues and inconsistencies. Let me know if there's anything missing. I ho...

Mastodonのフォークであるglitch-socに絵文字リアクション実装のPR、なお現在のところFedibirdとはAPIの互換性はない