R1:91a8a6d618cb

Initial cut of CalendarEditEngine

Summary:
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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9275

Differential Revision: https://secure.phabricator.com/D16235