FED Cohort assignments

@talbs Would you mind reviewing this?
What is this used for? This seems a bit flimsy as a state-based class and style rule. Can you leverage something like https://github.com/edx/edx-platform/blob/clrux/cohort-assignments-V3/lms%2Fstatic%2Fsass%2Fbase%2F_mixins.scss#L112 instead?