Ghost events should provide a link to original event

Summary: Closes T8356, Ghost events should provide a link to original event

Test Plan: Open an instance at /calendar/event/{id}/{index}. It should have a link to /calendar/event/{id}. Original recurring event should not link to itself.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8356

Differential Revision: https://secure.phabricator.com/D13068