Event detail page

There should be a detail page for an event, for event owners. It shiould display all relevant details of the event, plus the list of registrations for the event.

The general view that lists all registratiosn can then be removed.

The page should also provide access to group-based features in an easier way, e.g. to the additional fields. The additional fields for the linked group should be accessible and editable right from the event page.