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/components/compose_panel.js

@@ -5,6 +5,7 @@ import SearchContainer from 'mastodon/features/compose/containers/search_contain
 import ComposeFormContainer from 'mastodon/features/compose/containers/compose_form_container';
 import NavigationContainer from 'mastodon/features/compose/containers/navigation_container';
 import LinkFooter from './link_footer';
+import ServerBanner from 'mastodon/components/server_banner';
 import { changeComposing } from 'mastodon/actions/compose';
 
 export default @connect()
@@ -35,6 +36,7 @@ class ComposePanel extends React.PureComponent {
 
         {!signedIn && (
           <React.Fragment>
+            <ServerBanner />
             <div className='flex-spacer' />
           </React.Fragment>
         )}