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 times with minor changes, saw them reflected in the UI with transactions.

{F1870027}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16701