FED Cohort assignments

@talbs Would you mind reviewing this?
If you're abstracting this out, it looks there's still another ``.msg`` definition here (https://github.com/edx/edx-platform/blob/clrux/cohort-assignments-V3/lms/static/sass/course/instructor/_instructor_2.scss#L59). Those properties should be moved to the root your new rule that has the nested specific case rules included. Please delete the old definitions afterwards.