Added Instance Number to Ghost Events
Summary: Closes T8460.
Test Plan: In the Calendar application, create a recursive event. After editing a recursion of the event, verify that the "Recurrence of Event" property is of the form "<event sequence number> of <parent event>".
Reviewers: lpriestley, epriestley, Blessed Reviewers
Reviewed By: epriestley, Blessed Reviewers
Subscribers: epriestley, Korvin
Maniphest Tasks: T8460
Differential Revision: https://secure.phabricator.com/D13307