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.

To follow up, we don't enable {{baseUrl}} at the moment. To do so, add:

    $env->addExtension(new \Slim\Views\TwigExtension());

as the first line to the `initialise()` method in `app/src/View/Functions.php`.

(Though you should probably add a `use` rather than the fully qualified class name.)