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
`test_sections_can_be_locked` and `test_subsections_can_be_locked` now verify that all descendants are in a staff lock state.