ref(conference): change when the room reference is removed (#3808)

Delay removing the room reference. This is in case a
consumer of the API is attempting to submit feedback
after hangup but before redirecting to another page.
If the room reference is removed, feedback submission
will fail during this period.

conference.js

             requestFeedbackPromise,
             this.leaveRoomAndDisconnect()
         ]).then(values => {
+            this._room = undefined;
+            room = undefined;
+
             APP.API.notifyReadyToClose();
             maybeRedirectToWelcomePage(values[0]);
         });
         APP.store.dispatch(conferenceWillLeave(room));
 
         return room.leave()
-            .then(disconnect, disconnect)
-            .then(() => {
-                this._room = undefined;
-                room = undefined;
-            });
+            .then(disconnect, disconnect);
     },
 
     /**