user-groups: Create static/js/user_groups.js.

static/js/ui_init.js

@@ -249,6 +249,7 @@ $(function () {
     reload.initialize();
     server_events.initialize();
     people.initialize();
+    user_groups.initialize();
     unread.initialize();
     bot_data.initialize(); // Must happen after people.initialize()
     message_fetch.initialize();