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