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,

app/javascript/mastodon/reducers/rules.js

@@ -1,13 +0,0 @@
-import { RULES_FETCH_SUCCESS } from 'mastodon/actions/rules';
-import { List as ImmutableList, fromJS } from 'immutable';
-
-const initialState = ImmutableList();
-
-export default function rules(state = initialState, action) {
-  switch (action.type) {
-  case RULES_FETCH_SUCCESS:
-    return fromJS(action.rules);
-  default:
-    return state;
-  }
-}

app/javascript/mastodon/reducers/server.js

@@ -0,0 +1,19 @@
+import { SERVER_FETCH_REQUEST, SERVER_FETCH_SUCCESS, SERVER_FETCH_FAIL } from 'mastodon/actions/server';
+import { Map as ImmutableMap, fromJS } from 'immutable';
+
+const initialState = ImmutableMap({
+  isLoading: true,
+});
+
+export default function server(state = initialState, action) {
+  switch (action.type) {
+  case SERVER_FETCH_REQUEST:
+    return state.set('isLoading', true);
+  case SERVER_FETCH_SUCCESS:
+    return fromJS(action.server).set('isLoading', false);
+  case SERVER_FETCH_FAIL:
+    return state.set('isLoading', false);
+  default:
+    return state;
+  }
+}