R1:3e15e0b9806b

Store more datetime information on Calendar transactions and improve rendering behaviors

Summary:
Fixes T11805. Depends on D16785. This generally tries to smooth out transactions:

- All-day stuff now says "Nov 3" instead of "Nov 3 12:00:00 AM".
- Fewer weird bugs / extra transactions.
- No more silly extra "yeah, you definitely set that event time" transaction on create.

Test Plan: Edited events; changed from all-day to not-all-day and back again, viewed transaction log.

Reviewers: chad

Reviewed…