Merge branch 'dblessing_self_managed_sso_enforcement' into 'master'

Disable password authentication for SSO users

See merge request !168111



Merged-by: Drew Blessing <drew@gitlab.com>
Approved-by: Daniele Rossetti <drossetti@gitlab.com>
Approved-by: Harsha Muralidhar <hmuralidhar@gitlab.com>
Approved-by: Michał Zając <mzajac@gitlab.com>
Reviewed-by: Drew Blessing <drew@gitlab.com>
Reviewed-by: Andrew Evans <aevans@gitlab.com>
Reviewed-by: Bogdan Denkovych <bdenkovych@gitlab.com>
Reviewed-by: Daniele Rossetti <drossetti@gitlab.com>