Add "visible_to_staff_only" field to support staff locking in Studio.

STUD-1875

@andy-armstrong and @benmcmorran Please review.

@cpennington I have added to _has_access_descriptor in LMS' access.py. Can you either review or defer to someone else appropriate from the LMS team?
Did you skip i18n because it isn't shown in the UI? I think it would be more consistent to wrap it anyway.