Cohort Management: Add Group Static UI + Clean Up

This work addresses:

* adding in static UI rendering and styling for adding a new cohort group to the instructor dashboard 
* some loose ends with cohort management front end - syncing up markup and abstracting styling

This UI work can be viewed locally at - http://localhost:8000/template/ux/reference/instructor_dashboard/membership.html

I've also started a sandbox build and will update with a remote URL shortly.