R1:d4d46501ed1b

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.

Test Plan:
- 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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12488

Differential Revision: https://secure.phabricator.com/D17642