Show cohorts on the new instructor dashboard

TNL-161
That's a good idea. @explorerleslie or @talbs, can you suggest something.

There's a similar issue which is what we should show while the cohorts are still being fetched. Currently this section is hidden (because there are no cohorts yet) but it is confusing to have no loading indicator and seemingly the control has just vanished. In general, the instructor dashboard could use a pattern here, because a number of the sections appear unpopulated and then suddenly populate.