R1:ced151e6f2e8

Use transactions when importing events in Calendar, and update existing events

Summary:
Ref T10747.

- Apply what changes we can with transactions, so you can see how an event has changed and import actions are more explicit.
- I'll hide these from email/feed soon: I want them to appear on the event, but not generate notifications, since that could be especially annoying for automated events.
- When importing, try to update existing events if we can.

Test Plan:
Imported a ".ics" file several…