Implement forum cohort visibility labels correctly

...atic/coffee/spec/discussion/discussion_spec_helper.coffee

...c/discussion/view/discussion_thread_show_view_spec.coffee

common/static/js_test.yml

common/static/templates/discussion

common/templates/js/discussion/thread-show.underscore

lms/djangoapps/django_comment_client/base/views.py

lms/djangoapps/django_comment_client/forum/tests.py

lms/djangoapps/django_comment_client/forum/views.py

lms/djangoapps/django_comment_client/utils.py

lms/templates/discussion/_underscore_templates.html