Add a new tab to rooms with forward extremities (#107)

Add a new tab to rooms with forward extremities.

src/App.js

@@ -73,6 +73,7 @@ const App = () => (
     <Resource name="joined_rooms" />
     <Resource name="pushers" />
     <Resource name="servernotices" />
+    <Resource name="forward_extremities" />
     <Resource name="room_state" />
   </Admin>
 );