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