Show cohorts on the new instructor dashboard

TNL-161

Remove placeholder text
LMS: revises membership UI layout on instructor dashboard
LMS: adds static cohort group management UI to the Instructor Dashboard
TNL-160 Default cohorts, automatically created.
correcting needed markup for cohort management static UX template
addressing FED feedback
TNL-328 Server-side work for displaying type of cohort.
TNL-329 Client-side work for displaying cohort description.
Adding static rendering for cohort group creation UI

* adding styling/markup for "add new group" controls, messages, and form
* small FED and UI changes to overall cohort management UI
adding copy revisions and light styling support for cohort management
Support adding students to a cohort via the instructor dashboard.

TNL-163
converting instructor dashboard message UI to use .is-hidden vs.is-shown stateful CSS classes
LMS: revising .is-hidden utility class to be used at a global level
resolving display issues by re-implementing discussions-scoped is-hidden classes
Support adding cohorts from the instructor dashboard

TNL-162
Merge pull request #5407 from edx/cohort-management

Cohorts UI