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
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