Smooth over a few more transaction compatibility/structure issues with Calendar events

Summary: Ref T9275. This gets things roughly into shape for a cutover to EditEngine, mostly by fixing some problems with "recurrence end date" not being nullable while editing events.

Test Plan: Edited events with EditPro controller, nothing was obviously broken.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9275

Differential Revision: https://secure.phabricator.com/D16282