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/report/rules.js

@@ -7,7 +7,7 @@ import Button from 'mastodon/components/button';
 import Option from './components/option';
 
 const mapStateToProps = state => ({
-  rules: state.get('rules'),
+  rules: state.getIn(['server', 'rules']),
 });
 
 export default @connect(mapStateToProps)