FED Cohort assignments

@talbs Would you mind reviewing this?
This new rule seems redundant with the pre-existing one here (https://github.com/edx/edx-platform/blob/clrux/cohort-assignments-V3/lms/static/sass/course/instructor/_instructor_2.scss#L87). The styling is different, but the UI purpose is similar.

If you need this rule for your ``.inline`` work, I'd recommend nesting it inside of the ``.inline`` defiinition or prefixing this selector to ``.inline-error``.