Add Studio support for cohorted courseware

TNL-652
@catong - I've spoken to @andy-armstrong about this. I'm going with "component" for now. In the future, we'll want to support group visibility on units/subsections/sections, so we'll have to change it at that point. However, to achieve this, there will need to be both a significant refactor and a decision on category display name terminology since these validation messages will eventually be in both Studio and LMS, and I'm not sure it's worth the time at this point given our deadline.