subs: Allow filtering by is_user_active in get_active_subscriptions.

get_active_subscriptions_for_stream_id should allow specifying whether
subscriptions of deactivated users should be included in the result.
Active subs of deactivated users are  a subtlety that's easy to miss
when writing relevant code, so we make include_deactivated_users a
mandatory kwarg - this will force callers to definitely give thought to
whether such subs should be included or not.

This commit is just a refactoring, we keep original behavior everywhere
- there are places where subs of deactivates users should probably be
excluded but aren't - we don't fix that here, it'll be addressed in
follow-up commits.

zerver/lib/stream_topic.py

@@ -28,4 +28,6 @@ class StreamTopicTarget:
         return {row["user_profile_id"] for row in query}
 
     def get_active_subscriptions(self) -> QuerySet:
-        return get_active_subscriptions_for_stream_id(self.stream_id)
+        return get_active_subscriptions_for_stream_id(
+            self.stream_id, include_deactivated_users=True
+        )