Extract StreamTopicTarget.get_active_subscriptions().

Note that this code leads to a slightly different query, because
we join to one row in the small Recipient table to match
stream_id to recipient.type_id.

zerver/lib/actions.py

@@ -830,10 +830,7 @@ def get_recipient_info(recipient, sender_id, stream_topic, possibly_mentioned_us
         assert(len(message_to_user_ids) in [1, 2])
 
     elif recipient.type == Recipient.STREAM:
-        subscription_rows = Subscription.objects.filter(
-            recipient=recipient,
-            active=True,
-        ).values(
+        subscription_rows = stream_topic.get_active_subscriptions().values(
             'user_profile_id',
             'push_notifications',
             'in_home_view',

zerver/lib/stream_topic.py

@@ -1,6 +1,13 @@
 from typing import (Dict, List, Text, Set)
+from django.db.models.query import QuerySet
 
-from zerver.models import MutedTopic
+from zerver.lib.stream_subscription import (
+    get_active_subscriptions_for_stream_id,
+)
+
+from zerver.models import (
+    MutedTopic,
+)
 
 class StreamTopicTarget(object):
     '''
@@ -26,3 +33,7 @@ class StreamTopicTarget(object):
             row['user_profile_id']
             for row in query
         }
+
+    def get_active_subscriptions(self):
+        # type: () -> QuerySet
+        return get_active_subscriptions_for_stream_id(self.stream_id)