Add helper method on event object to determine if event is the parent of a recurrence.

Summary: Ref T8472, Add helper method on event object to determine if event is the parent of a recurrence.

Test Plan: No user facing change.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T8472

Differential Revision: https://secure.phabricator.com/D13221