Migrate Calendar away from stored-epoch fields

Summary:
Ref T10747. This deprecates "dateFrom", "dateTo", "allDayDateFrom", "allDayDateTo", and "recurrenceEndDate".

They are replaced with "utc*Epoch" fields (for querying) and CalendarDateTime objects (for start, end, until). These objects can represent the full range of dates and times expressible in ICS format, allowing us to import a wider range of ICS events.

Test Plan:
Ran migrations, viewed/edited Calendar, didn't catch anything catastrophcially broken.

This likely needs some followups, I'll keep it local for a bit until I'm confident I didn't break anything too catastrophically. I'm retaining the old data for now so we can likely fix things if it turns out there is some sort of issue.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16664