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: vrana, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D3186