don't show 'user has left' notification for hidden users

conference.js

         });
 
         room.on(JitsiConferenceEvents.USER_LEFT, (id, user) => {
+            if (user.isHidden()) {
+                return;
+            }
             APP.store.dispatch(participantLeft(id, user));
             logger.log('USER %s LEFT', id, user);
             APP.API.notifyUserLeft(id);