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/actions/rules.js

@@ -1,27 +0,0 @@
-import api from '../api';
-
-export const RULES_FETCH_REQUEST = 'RULES_FETCH_REQUEST';
-export const RULES_FETCH_SUCCESS = 'RULES_FETCH_SUCCESS';
-export const RULES_FETCH_FAIL    = 'RULES_FETCH_FAIL';
-
-export const fetchRules = () => (dispatch, getState) => {
-  dispatch(fetchRulesRequest());
-
-  api(getState)
-    .get('/api/v1/instance').then(({ data }) => dispatch(fetchRulesSuccess(data.rules)))
-    .catch(err => dispatch(fetchRulesFail(err)));
-};
-
-const fetchRulesRequest = () => ({
-  type: RULES_FETCH_REQUEST,
-});
-
-const fetchRulesSuccess = rules => ({
-  type: RULES_FETCH_SUCCESS,
-  rules,
-});
-
-const fetchRulesFail = error => ({
-  type: RULES_FETCH_FAIL,
-  error,
-});

app/javascript/mastodon/actions/server.js

@@ -0,0 +1,30 @@
+import api from '../api';
+import { importFetchedAccount } from './importer';
+
+export const SERVER_FETCH_REQUEST = 'Server_FETCH_REQUEST';
+export const SERVER_FETCH_SUCCESS = 'Server_FETCH_SUCCESS';
+export const SERVER_FETCH_FAIL    = 'Server_FETCH_FAIL';
+
+export const fetchServer = () => (dispatch, getState) => {
+  dispatch(fetchServerRequest());
+
+  api(getState)
+    .get('/api/v2/instance').then(({ data }) => {
+      if (data.contact.account) dispatch(importFetchedAccount(data.contact.account));
+      dispatch(fetchServerSuccess(data));
+    }).catch(err => dispatch(fetchServerFail(err)));
+};
+
+const fetchServerRequest = () => ({
+  type: SERVER_FETCH_REQUEST,
+});
+
+const fetchServerSuccess = server => ({
+  type: SERVER_FETCH_SUCCESS,
+  server,
+});
+
+const fetchServerFail = error => ({
+  type: SERVER_FETCH_FAIL,
+  error,
+});