R1:d3fc1800f86a

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…