EAS AD may require username.

Summary:
1. Console auth flow permits username to be entered for password_auth.
2. Migration to add easaccount.username easaccount.eas_auth columns.

Test Plan:
Run the console auth flow.
Run the migration on test db.

Reviewers: khamidou, emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D554