Add zerver/lib/stream_subscription.py.

The first method we extract to this library is
get_active_subscriptions_for_stream_id().

We also move num_subscribers_for_stream_id() to here, which
is slightly annoying (having the method on Stream was nice)
but avoids some circular dependency issues.

zerver/lib/stream_subscription.py

@@ -0,0 +1,19 @@
+from django.db.models.query import QuerySet
+from zerver.models import (
+    Recipient,
+    Subscription,
+)
+
+def get_active_subscriptions_for_stream_id(stream_id):
+    # type: (int) -> QuerySet
+    return Subscription.objects.filter(
+        recipient__type=Recipient.STREAM,
+        recipient__type_id=stream_id,
+        active=True,
+    )
+
+def num_subscribers_for_stream_id(stream_id):
+    # type: (int) -> int
+    return get_active_subscriptions_for_stream_id(stream_id).filter(
+        user_profile__is_active=True,
+    ).count()