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