FED Cohort assignments

@talbs Would you mind reviewing this?
This ruleset seems too complicated. Other actions added as part of this UI have used simpler rules (https://github.com/edx/edx-platform/blob/master/lms%2Fstatic%2Fsass%2Fcourse%2Finstructor%2F_instructor_2.scss#L535). Can you follow suit here or abstract something out that works in multiple cases?