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
And verifyImplicitStaffOnly(false)?

Or perhaps verifyStaffOnly(false) should check both verifyImplicitStaffOnly(false) and verifyExplicitStaffOnly(false)?