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
I don't believe so. `isStaffOnly` is true when we're requesting explicit staff lock, and false if we're requesting to remove explicit staff lock. Although, now that I look at this line, I realize it should really be the following:

```
has_explicit_staff_lock: isStaffOnly,
```