Support "Edit just this event" on the parent event in a series

Summary:
Ref T11804. This one is messy because we have to fork the next event, possibly creating it first.

Then we can edit the parent normally.

Test Plan: Cancelled the first event in a series, only that one cancelled.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11804

Differential Revision: https://secure.phabricator.com/D16781