Add server banner to web app, add GET /api/v2/instance to REST API (#19294)

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,
+});