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
Merge pull request #5325 from edx/talbs/cohort-management-addgroup

Cohort Management: Add Group Static UI + Clean Up
Support adding students to a cohort

TNL-163
Bok choy test for moving students.