Adds possibility to edit an event in web2

This commit adds the possibility to edit an event via the web2 interface
Its required by Symfonys PropertyAccessor. Without the ```getHref()``` you'll get a 
     
    Slim Application Error
    
    The application could not run because of the following error:
    Details
    Type: Symfony\Component\PropertyAccess\Exception\NoSuchPropertyException
    Message: Neither the property "href" nor one of the methods "getHref()", "href()", "isHref()", "hasHref()", "__get()" exist and have public access in class "Event\EventEntity".
    File: /vagrant/joindin-web2/vendor/symfony/property-access/Symfony/Component/PropertyAccess/PropertyAccessor.php
    Line: 367