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
As discussed, use parent_xblock to avoid looking up the parent. This means the method probably isn't needed at all (just check parent.visible_to_staff directly).