Joindin 441: Redirecting the I'm attending button

Would someone be able to review my changes to Controller/Event.php, I'm not convinced that the way I'm doing the redirecting is the best way. If anyone can suggest something better that would be great.

This configuration option is not included in this PR and causes code to fail:

Fatal error: Uncaught exception 'ErrorException' with message 'Undefined index: localUrl' in /vagrant/joindin-web2/web/index.php on line 54
( ! ) ErrorException: Undefined index: localUrl in /vagrant/joindin-web2/web/index.php on line 54