Remove all readers and writers of "accountID" on "ExternalAccount"

Summary: Depends on D21019. Ref T13493. There are no more barriers to removing readers and writers of "accountID"; the new "ExternalAccountIdentity" table can replace it completely.

Test Plan: Linked and unlinked OAuth accounts, logged in with OAuth accounts, tried to double-link OAuth accounts, grepped for affected symbols.

Maniphest Tasks: T13493

Differential Revision: https://secure.phabricator.com/D21022