R1:05eb16d6de51

Update unusual handling of external accounts in "Password" auth provider

Summary:
Depends on D21013. Ref T13493. When users log in with most providers, the provider returns an "ExternalAccount" identifier (like an Asana account GUID) and the workflow figures out where to go from there, usually a decision to try to send the user to registration (if the external account isn't linked to anything yet) or login (if it is).

In the case of password providers, the password is really a property of an existing…