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?
Yes, I should wrap it anyway. I was thinking it wouldn't be shown to students (I purposefully did not put a display name, but I thought the help text was helpful for those reading the code). Of course, I then forget to exclude the property from Advanced Settings, so I will also make that change.