Merge branch 'main' into glitch-soc/merge-upstream

Conflicts:
- `README.md`:
  Upstream updated its README, while we have a completely different one.
  Kept our README.
- `app/controllers/concerns/web_app_controller_concern.rb`:
  Conflict because of glitch-soc's theming system.
  Additionally, glitch-soc has different behavior regarding moved accounts.
  Ported some of the changes, but kept our overall behavior.
- `app/javascript/packs/admin.js`:
  Code changes actually applied to `app/javascript/core/admin.js`

app/javascript/mastodon/features/compose/components/compose_form.js

@@ -226,7 +226,7 @@ class ComposeForm extends ImmutablePureComponent {
 
         <ReplyIndicatorContainer />
 
-        <div className={`spoiler-input ${this.props.spoiler ? 'spoiler-input--visible' : ''}`} ref={this.setRef}>
+        <div className={`spoiler-input ${this.props.spoiler ? 'spoiler-input--visible' : ''}`} ref={this.setRef} aria-hidden={!this.props.spoiler}>
           <AutosuggestInput
             placeholder={intl.formatMessage(messages.spoiler_placeholder)}
             value={this.props.spoilerText}