FED Cohort assignments

@talbs Would you mind reviewing this?
This seems a bit of a brittle Sass placeholder. How many times are you referencing this elsewhere? If its few, its probably better to add the prop/value to those rules. Also, can this leverage https://github.com/edx/edx-platform/blob/master/lms%2Fstatic%2Fsass%2Fcourse%2Finstructor%2F_instructor_2.scss#L535 instead?