FED Cohort assignments

@talbs Would you mind reviewing this?
Having this class extend the ``%no-margin-top`` placeholder defeats the purpose of the having the placeholder in the first place (http://thesassway.com/intermediate/understanding-placeholder-selectors). 

My main issues are 1)this  is an extra rule in the style sheet here and 2) this is also causing us to add ``.no-margin-top`` classes in our DOM/HTML. I'd prefer our classes in the DOM to be as semantic as possible and in the case where they serve more specific layout/UI needs, we are not getting as specific as a margin-top override.