Store "start", "end", and "until" event dates as CalendarDateTime objects

Summary:
Ref T10747. This does double-writes and starts generating/writing CalendarDateTimes.

This greater flexibility is necessary to support the full range of ICS-specifiable events, including "floating" events.

This doesn't do anything yet.

Test Plan: Created and edited events, verified sensible representations of corresponding datetimes appeared in the database.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16661