Summary: When you edit "X and all future events", X becomes the new parent of an event series. Currently, it loses its relationship to its original parent. Instead, retain that relationship -- it's separate from the normal "parent", but we can use it to make the UI more clear or tweak behaviors later. This mostly just keeps us from losing/destroying data that we might need/want later. Test Plan: - Ran migrations. - Cancelled "X and all future events", saw sensible-appearing beahvior in the database for "seriesParentPHID". Reviewers: chad Reviewed By: chad Differential Revision: https://secure.phabricator.com/D16780