Support adding cohorts from the instructor dashboard

TNL-162
This was just a cut-and-paste from your code in cohort_editor! I had the same thought, so this is how I have it after working on it last night:

            var model = new NotificationModel(
                {
                    type: 'warning',
                    title: title,
                    message: message,
                    actionText: gettext('Add Cohort Group'),
                    actionClass: 'action-create',
                    actionIconClass: 'icon-plus'
                },
                {parse: true}
            );

I'll make the same change to cohort_editor.