Implement forum cohort visibility labels correctly

TNL-25
This should use `<%-`. Also, to replicate the ongoing conversation about this line that has now disappeared (thanks, GitHub!):

From @gwprice:
We definitely don't want to show this unless the course is cohorted. We should figure out if we want to show it at all or only label things with limited visibility.

From @andy-armstrong:
I discussed this with @explorerleslie a couple of weeks back, and she wanted to always show it, even for non-cohorted courses.

From @explorerleslie:
Yeah, I'm fine if this shows to everyone. I think it's helpful for course staff that may be working on courses that are cohorted and not, to see a visibility indicator always.

From @gwprice:
On the other hand, it might confuse course staff who are not working on any cohorted course that might wonder why it's there and how one would go about making a thread that is not visible to everyone.