Record a "series parent PHID" on Calendar events that retains relationships after forks

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