unread_msgs: Fix all unreads in muted stream being treated as muted.

Earlier, 'is_row_muted' returned 'true' if the message was in
a muted stream or muted topic.

If the message is in an unmuted or followed topic in a muted
stream, such topics should be treated as not muted topics
in an unmuted stream.

This commit fixes the incorrect behavior.

Now, for wildcard mentions, 'unread_msgs.mentions' exclude
the IDs in muted streams only if the message is in default or
muted topic.

Also, 'unread_msgs.count' takes into account the unreads in unmuted
or followed topics in muted streams too.

Documents that this bug was fixed in the API changelog.

zerver/lib/user_topics.py

@@ -109,6 +109,21 @@ def set_topic_visibility_policy(
         )
 
 
+def get_topic_visibility_policy(
+    user_profile: UserProfile,
+    stream_id: int,
+    topic_name: str,
+) -> int:
+    try:
+        user_topic = UserTopic.objects.get(
+            user_profile=user_profile, stream_id=stream_id, topic_name__iexact=topic_name
+        )
+        visibility_policy = user_topic.visibility_policy
+    except UserTopic.DoesNotExist:
+        visibility_policy = UserTopic.VisibilityPolicy.INHERIT
+    return visibility_policy
+
+
 @transaction.atomic(savepoint=False)
 def bulk_set_user_topic_visibility_policy_in_database(
     user_profiles: List[UserProfile],