Migrate all "accountID" values to "ExternalAccountIdentifier" objects

Summary: Depends on D21016. Ref T13493. This copies existing external account "accountID" values into the "ExternalAccountIdentifier" table, preparing for an authority switch.

Test Plan: Ran migration several times, looked at the data that came out of it, saw sensible results. Logged out / in with external accounts.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13493

Differential Revision: https://secure.phabricator.com/D21017