Add server banner to web app, add GET /api/v2/instance to REST API (#19294)

Cherry-picked d2528b26b6

Conflicts:
- `app/serializers/initial_state_serializer.rb`:
  Upstream changed stuff, we had extra attributes.
  Applied upstream changes while keeping our extra attributes.
- `app/serializers/rest/instance_serializer.rb`:
  Upstream actually moved that to `app/serializers/rest/v1/instance_serializer.rb`,
  so updated that file by keeping our extra attributes, and took upstream's
  version of `app/serializers/rest/instance_serializer.rb`.
- `spec/views/about/show.html.haml_spec.rb`:
  Took upstream's version.

app/javascript/mastodon/features/ui/index.js

@@ -13,7 +13,7 @@ import { debounce } from 'lodash';
 import { uploadCompose, resetCompose, changeComposeSpoilerness } from '../../actions/compose';
 import { expandHomeTimeline } from '../../actions/timelines';
 import { expandNotifications } from '../../actions/notifications';
-import { fetchRules } from '../../actions/rules';
+import { fetchServer } from '../../actions/server';
 import { clearHeight } from '../../actions/height_cache';
 import { focusApp, unfocusApp, changeLayout } from 'mastodon/actions/app';
 import { synchronouslySubmitMarkers, submitMarkers, fetchMarkers } from 'mastodon/actions/markers';
@@ -389,7 +389,7 @@ class UI extends React.PureComponent {
       this.props.dispatch(expandHomeTimeline());
       this.props.dispatch(expandNotifications());
 
-      setTimeout(() => this.props.dispatch(fetchRules()), 3000);
+      setTimeout(() => this.props.dispatch(fetchServer()), 3000);
     }
 
     this.hotkeys.__mousetrap__.stopCallback = (e, element) => {