Initial cut of CalendarEditEngine
Ref T9275. This builds a Calendar EditEngine which only edits "name".
I'll add more fields, Conduit, etc., and move to modular transactions in future changes.
Test Plan: Used editpro/ URI manually to edit the name of an event.
Reviewed By: chad
Maniphest Tasks: T9275
Differential Revision: https://secure.phabricator.com/D16235