Summary:
This also updates Authorizer to link the authSubject
to Grist users if not previously linked. Linked subjects
are now used as the username for password-based logins,
instead of emails, which remain as a fallback.
Test Plan: Existing tests, and tested login flows manually.
Reviewers: paulfitz
Reviewed By: paulfitz
Differential Revision: https://phab.getgrist.com/D3356