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.

Shouldn't $event and $user be parameters of the `confirm()` method? The confirm method should be repeatable where you confirm a user with an event where as the $apiEvent is an immutable in this object because it forms the bonds between this class and the API.

With this change you can even inject the EventAttendance class using a DIC should the $apiEvent be loadable from the DIC.