Add a repair migration to fix legacy Calendar events without "utcInstanceEpoch"
Summary: Fixes T12488. Some events appear to have survived earlier migrations without getting completely fixed. Fix them.
- Ran migration locally with bin/storage upgrade (but: I could not reproduce this problem locally).
- Ran migration in production and saw ICS import stop fataling.
Reviewed By: chad
Maniphest Tasks: T12488
Differential Revision: https://secure.phabricator.com/D17642