Assign cohorts to groups in the instructor dashboard

TNL-653
I'm all for this philosophical UI debate and coming to a set of patterns/state management that we're all comfortable with - I don't think the cohort management UI within the Instructor Dashboard views is the place to fall on any sword.

@andy-armstrong, if we're already using the ``is-hidden`` approach for UI you guys have wired through in this focused case/feature, let's stick with that. If not, I'd prefer to run with how you'd like to set it up for now.

@clrux, let's make a note of this type of case and roll it into the pattern library work that is forthcoming.