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/reducers/index.js

@@ -17,7 +17,7 @@ import status_lists from './status_lists';
 import mutes from './mutes';
 import blocks from './blocks';
 import boosts from './boosts';
-import rules from './rules';
+import server from './server';
 import contexts from './contexts';
 import compose from './compose';
 import search from './search';
@@ -62,7 +62,7 @@ const reducers = {
   mutes,
   blocks,
   boosts,
-  rules,
+  server,
   contexts,
   compose,
   search,