Implement drag-and-drop support

Problem/Motivation

The current version correctly sets the "editable" attribute on events, but no changes are actually saved to the database.

Proposed resolution

Create an endpoint that will properly process changes, including handling overrides for recurring Smart Date events. For the latter, it may make most sense to leverage the controller that was added to Smart Date to handle similar updates from Fullcalendar View. Note that this code in turn relies on code in Smart Date's FullCalendarViewProcessor plugin which constructs an 'eid' value based on whether or not it is an instance of a recurring event.