adding copy revisions and light styling support for cohort management
Merge pull request #5390 from edx/talbs/cohort-management-uifixes

Cohort Management: Light UI Fixes
Support adding students to a cohort

TNL-163
Bok choy test for moving students.
Make removing old notifications less flashy.
Minor cleanup (to be squashed).
Minor cleanup (to be squashed).
Minor cleanup (to be squashed).
Fix broken import.
Apply code review comments