stream_topic: Refactor user_ids_muting_topic.

This refactors and renames user_ids_muting_topic to accept a parameter
'visibility_policy' and fetch user IDs that have a specific
visibility_policy(provided as the parameter) set for a topic.

zerver/lib/stream_topic.py

@@ -15,11 +15,11 @@ class StreamTopicTarget:
         self.stream_id = stream_id
         self.topic_name = topic_name
 
-    def user_ids_muting_topic(self) -> Set[int]:
+    def user_ids_with_visibility_policy(self, visibility_policy: int) -> Set[int]:
         query = UserTopic.objects.filter(
             stream_id=self.stream_id,
             topic_name__iexact=self.topic_name,
-            visibility_policy=UserTopic.MUTED,
+            visibility_policy=visibility_policy,
         ).values(
             "user_profile_id",
         )