chore: disable forum digest subscription section by default (#31459)

Current State of the feature

Initially, this feature used the Notifier Django app but was deprecated
and removed from the configuration in favor of edx-notifications.
More info about this deprecation can be found in the GitHub issue (
openedx/wg-build-test-release#22).

The edx-notifications app was not supported though, so its repository
was archived and moved to openedx-unsupported/edx-notifications.
Check the deprecation ticket (openedx-unsupported/edx-notifications#253)
and corresponding discussion (https://discuss.openedx.org/t/deprecation-removal-edx-notifications-repository/6748)
for details.

What remains in the edx-platform

The forum initial page contains the how-to info with the notification
preferences toggle.

This section is responsible for toggling the "notification_pref" user
preference (aka NOTIFICATION_PREF_KEY). I didn't find any usage of this
preference across the platform, so I'm suggesting hiding it by default
by setting the ENABLE_FORUM_DAILY_DIGEST setting to `False`.

lms/djangoapps/discussion/config/settings.py

@@ -5,7 +5,7 @@
 
 # .. toggle_name: FEATURES['ENABLE_FORUM_DAILY_DIGEST']
 # .. toggle_implementation: DjangoSetting
-# .. toggle_default: True
+# .. toggle_default: False
 # .. toggle_description: Settings for forums/discussions to on/off daily digest
 #   feature. Set this to True if you want to enable users to subscribe and unsubscribe
 #   for daily digest. This setting enables deprecation of daily digest.
@@ -16,4 +16,4 @@
 
 def is_forum_daily_digest_enabled():
     """Returns whether forum notification features should be visible"""
-    return settings.FEATURES.get('ENABLE_FORUM_DAILY_DIGEST', True)
+    return settings.FEATURES.get('ENABLE_FORUM_DAILY_DIGEST', False)

lms/djangoapps/discussion/notification_prefs/tests.py

@@ -28,6 +28,7 @@ class NotificationPrefViewTest(UrlResetMixin, TestCase):  # lint-amnesty, pylint
     INITIALIZATION_VECTOR = b"\x00" * 16
 
     @patch.dict("django.conf.settings.FEATURES", {"ENABLE_DISCUSSION_SERVICE": True})
+    @patch.dict("django.conf.settings.FEATURES", {"ENABLE_FORUM_DAILY_DIGEST": True})
     def setUp(self):
         super().setUp()
         self.user = UserFactory.create(username="testuser")