Cohort assignment

FED for [cohort assignments] (https://openedx.atlassian.net/browse/UX-1086). I used the instructor dashboard as a starting point, including the .html template and .scss. No JS here, but let me know if you'd like it included. Some of the markup is commented out allowing focus to be on the *additions*, as the .html file is quite large.

(cc @talbs)