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

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;
+  }
+}