Convert Calendar to Modular Transactions

Summary:
Ref T9275. Swaps Calendar over to modular transactions. Theoretically, this has almost no effect on anything.

Ref T10633. I didn't actually do anything here yet, but this gets us ready to put timestamps in email.

Test Plan: Created and edited a bunch of events, nothing seemed catastrophically broken.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9275, T10633

Differential Revision: https://secure.phabricator.com/D16286