Convert Calendar to Modular Transactions
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.
Reviewed By: chad
Differential Revision: https://secure.phabricator.com/D16286