Fixes #5186

On socket.io connection, all clients join a room pertaining to
their express session id. We use this room to keep track of any
sessions in different browser windows (but the same cookie jar),
so if a login/logout occurs, we can throw a session mismatch
modal.

This room can also be used to emit messages across windows/tabs...

src/socket.io/index.js

@@ -57,6 +57,7 @@ var ratelimit = require('../middleware/ratelimit');
 			socket.join('online_guests');
 		}
 
+		socket.join('sess_' + socket.request.signedCookies[nconf.get('sessionKey')]);
 		io.sockets.sockets[socket.id].emit('checkSession', socket.uid);
 	}