Homestyx hydra

R1:208f8ed526a1

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
Repository: R1 hydra
Commit Date: Oct 31 2016