Merge commit 'b0780cfeeda641645ea65da257a72ec507e71647' into glitch-soc/merge-upstream

Conflicts:
- `app/javascript/mastodon/load_locale.js`:
  The file moved to `app/javascript/mastodon/locales/load_locale.ts`.
  Ported the changes there and deleted `app/javascript/mastodon/load_locale.js`.
- `app/javascript/mastodon/locales/index.js`:
  The file moved to `app/javascript/mastodon/locales/index.ts`.
  Did *not* port the changes as I want to try something a bit different.

package.json

@@ -50,6 +50,7 @@
     "abortcontroller-polyfill": "^1.7.5",
     "atrament": "0.2.4",
     "arrow-key-navigation": "^1.2.0",
+    "async-mutex": "^0.4.0",
     "autoprefixer": "^10.4.14",
     "axios": "^1.4.0",
     "babel-loader": "^8.3.0",