Hide sections when a learners cohort doesn't have access to the associated content group.

@MHaton mentioned that Sections are displayed for learners even when the learners cohort and the sections content group specify it's not for them.

Also annoyingly, learners always have access to sections, even when there are no units assigned to them in those sections. This means you'd have to put something in there saying "This isn't for you, move along". It's not ideal.

@ormsbee thought that the same logic that is used to hide subsections could be extend to hide sections. That logic is here.