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
It's confusing because if the xblock itself is locked, that IS the source of the lock (no matter what the inherited locks above are). 

If you need to keep ignore, change the name of the method to reflect that it is finding the inherited lock source.