Merge: contrib/opportunity: store local participant's name in a cookie

So we don't have to write our own name every single time we participate to an event.

Already deployed on xymus.net, you can test it here: http://xymus.net/opportunity/?meetup_id=5E8F7A00589D5F567FFB07EA09A8630A8A2BA048

Pull-Request: #1765
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>