Assign cohorts to groups in the instructor dashboard

TNL-653
I've updated this in the latest commit.

I'm torn here. Part of me wants to stick with convention, but the other part of me thinks that it depends on the scenario. In those cases where elements are visible and programmatically hidden, `.is-hidden` would be applied and would make sense. But for those cases where the elements are shown programmatically, then I think `.is-visible` is more semantic. But it also depends on how one is looking at the code - from a front-end or back-end viewpoint. Happy to discuss this further.