Give "PhabricatorUserEmail" a PHID

Summary:
Ref T13444. To interact meaningfully with "DestructionEngine", objects need a PHID. The "UserEmail" object currently does not have one (or a real "Query").

Provide basic PHID support so "DestructionEngine" can interact with the object more powerfully.

Test Plan:

• Ran migrations, checked data in database, saw sensible PHIDs assigned.
• Added a new email address to my account, saw it get a PHID.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13444

Differential Revision: https://secure.phabricator.com/D20913