R1:f9fcaa1f8447

Migrate project membership to edges

Summary:
- Store project members in edges.
- Migrate existing members to edge storage.
- Delete PhabricatorProjectAffiliation.
- I left the actual underlying data around just in case something goes wrong; we can delete it evenutally.

Test Plan:
- Ran migration.
- Created a new project.
- Joined and left a project.
- Added and removed project members.
- Manually called PhabricatorOwnersOwner::loadAffiliatedUserPHIDs() to verify its behavior.

Reviewers…