WIP - Enable or disable staff lock from course outline

STUD-1851
This line has been bugging me - I assume that we don't use [`xblock.get_parent()`](https://xblock.readthedocs.org/en/latest/api/xblock.html#xblock.core.XBlock.get_parent) so that we can pass in the draft_preferred parameter, right? Can someone explain why that parameter is necessary? Is it because draft components shadow published components on the unit page?