Assign RepositoryIdentity objects to commits

Summary: Depends on D19429. Depends on D19423. Ref T12164. This creates new columns authorIdentityPHID and committerIdentityPHID on commit objects and starts populating them. Also adds the ability to explicitly set an Identity's assignee to "unassigned()" to null out an incorrect auto-assign. Adds more search functionality to identities. Also creates a daemon task for handling users adding new email address and attempts to associate unclaimed identities.

Test Plan: Imported some repos, watched new columns get populated. Added a new email address for a previous commit, saw daemon job run and assign the identity to the new user. Searched for identities in various and sundry ways.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T12164

Differential Revision: https://secure.phabricator.com/D19443