Assign cohorts to groups in the instructor dashboard

TNL-653
Once this work is put back into ``_instructor_2.scss``, I still have concerns around this new type of message styling. How will it play with the already defined ``.msg-error`` (https://github.com/edx/edx-platform/blob/christina/link_group_content_to_cohort/lms/static/sass/course/instructor/_instructor_2.scss#L72)?