Improve admin interface for managing occurrences

[jmurty]

The admin interface for Events is fairly rudimentary and uses plain inline formsets for the repeat generators and occurrences associated with an event.

A simple inline formset will be difficult to work with for repeating events that may have dozens, or even hundreds, of occurrences. An admin user may need to find and modify specific occurrences within the set, and doing so will become more difficult once there are more than a few related occurrences.

[jmurty]

Here is a screenshot of the basic admin currently in place: