Add flag to indicate event needs moderation

It would be nice if users could flag an event needing moderation. Easiest way is probably to have a button on the event details view and a text field for a short description of the problem.

Need to update:
- EventModel with a flag field (Integer/Bool) and a flag_description field (Char).
- EventAdmin with list_filter for the flag field.
- event_detail.html
