Assign cohorts to groups in the instructor dashboard

TNL-653
For the sake of getting this finite Q2 task over the line and that this UI should be scoped to the Instructor Dashboard that abstraction task can wait.

What I'm asking for here is that if you need an inline error message, try to weave it into the already created messages in this Sass and Instructor Dashboard UI (adding additional rules or overriding properties as you need them).