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.

The same comment as with https://github.com/joindin/joindin-web2/pull/102/files#diff-c698983d47343fe87d715beb43d1f6f5R166 applies here; $event may return an item that is not an event; in which case the action will error