Add ability to set staff lock from course outline

STUD-1851. @cahrens please review.

Sandbox available at http://studio.benmcmorran.m.sandbox.edx.org/course/edX/Open_DemoX/edx_demo_course
@mhoeber Did you see this wording? I don't think we should use "Staff Lock" here since we don't use that language in general.

And I wonder if we should have a special message for when an explicit staff lock is being set ("Explicitly Hiding Unit from Students...?).