Delete event and event groups

* [x] page: delete an event
* [x] page: delete an event group
* [x] event group deletion: update all events in group to remove the group
* [x] event deletion: rule, make sure that the event is not in the past
* [x] update event form edition to include link to delete page
* [x] update event group edition to include link to delete page

See #65 

'is_editing' or 'editing_event' would be a better name for this imo