Write ExternalAccountIdentifiers when interacting with external authentication providers

Summary:
Depends on D21015. When we sync an external account and get a list of account identifiers, write them to the database.

Nothing reads them yet and we still write "accountId", this just prepares us for reads.

Test Plan: Linked, refreshed, unlinked, and re-linked an external account. Peeked at the database and saw a sensible-looking row.

Differential Revision: https://secure.phabricator.com/D21016