EventIndexPanel: Fix the removal of the updateCurrentRoom listeners.

The event listeners that update the current room and try to get the
current stats from the event index don't seem to be unmounted because
this != this confusion.

Turning them into arrow methods of the react class fixes this.

src/async-components/views/dialogs/eventindex/ManageEventIndexDialog.js

@@ -46,7 +46,7 @@ export default class ManageEventIndexDialog extends React.Component {
         };
     }
 
-    updateCurrentRoom = async (room) => {
+    updateCurrentRoom = async(room) => {
         const eventIndex = EventIndexPeg.get();
         const stats = await eventIndex.getStats();
         let currentRoom = null;