R1:0872051bfa28

Make AuthProvider, ExternalAccount, and ExternalAccountIdentifier all Destructible

Summary: Depends on D21014. Ref T13493. Make these objects all use destructible interfaces and destroy sub-objects appropriately.

Test Plan:
- Used `bin/remove destroy --trace ...` to destroy a provider, a user, and an external account.
- Observed destruction of sub-objects, including external account identifiers.

Maniphest Tasks: T13493

Differential Revision: https://secure.phabricator.com/D21015