Implement a new configuration system for discussions plugins

Additional details available in the attached decisions document.

Co-authored-by: Kshitij Sobti <kshitij@sobti.in>
Co-authored-by: stvn <stvn@mit.edu>

cms/envs/common.py

@@ -1376,6 +1376,7 @@ INSTALLED_APPS = [
 
     # Discussion
     'openedx.core.djangoapps.django_comment_common',
+    'openedx.core.djangoapps.discussions',
 
     # for course creator table
     'django.contrib.admin',