More event nesting levels

At the moment, there appears to be two 'nesting levels' allowed by the system: events that have no parents and events that have no children. That is, one does not seem to be able to create a child event of a child event.

This additional layer of nesting is useful in an academic conference setting in which one has multiple simultaneous events which each have several live sessions and a "poster session," i.e. a time-slot in which a larger number of people present simultaneously in their own group meetings. The most natural structure for this is to have the posters as children of the poster session which is a child of that particular event.

Although arbitrarily deep nesting is likely bad for user experience more generally, this one additional level has a very defined and common need in large academic conference organization.