refactor: Convert discussions to pluggable app

This minimizes our footprint outside of the djangoapp, now and moving
forward. Not only can we drop the `lms/envs/common.py` change, but we
can also avoid touching `lms/urls.py` when we add the API. Everything
can stay contained within `openedx/core/djangoapps/discussions`.

openedx/core/djangoapps/discussions/apps.py

@@ -2,6 +2,8 @@
 Configure the django app
 """
 from django.apps import AppConfig
+from edx_django_utils.plugins import PluginSettings
+from edx_django_utils.plugins import PluginURLs
 
 
 class DiscussionsConfig(AppConfig):
@@ -9,3 +11,9 @@ class DiscussionsConfig(AppConfig):
     Configure the discussions django app
     """
     name = 'openedx.core.djangoapps.discussions'
+    plugin_app = {
+        PluginURLs.CONFIG: {
+        },
+        PluginSettings.CONFIG: {
+        },
+    }