R1:d69a7360ea17

Use DestructionEngine to destroy UserEmail objects

Summary: Ref T13444. Prepare to hook identity updates when user email addreses are destroyed.

Test Plan:
- Destroyed a user with `bin/remove destroy ... --trace`, saw email deleted.
- Destroyed an email from the web UI, saw email deleted.

Maniphest Tasks: T13444

Differential Revision: https://secure.phabricator.com/D20912