Assign PHIDs to calendar events

Summary: Ref T4375. We're going to need these for a bunch of infrastructure to work.

Test Plan: Ran migrations, checked DB, used `phid.query`.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T4375

Differential Revision: https://secure.phabricator.com/D8151