Simplify bulk_remove_subscriptions().

We extract get_bulk_stream_subscriber_info() from this
function to remove some of the complexity.  Also, in that
new function we avoid a hop to the database by querying
on stream ids instead of recipient ids.  The query that
gets changed here does require a join to the recipient
table (to get the stream id), so it's a little bit of a
tradeoff.

zerver/lib/stream_subscription.py

@@ -1,8 +1,10 @@
-from typing import List
+from typing import Dict, List, Tuple
+from mypy_extensions import TypedDict
 
 from django.db.models.query import QuerySet
 from zerver.models import (
     Recipient,
+    Stream,
     Subscription,
     UserProfile,
 )
@@ -37,6 +39,36 @@ def get_stream_subscriptions_for_users(user_profiles):
         recipient__type=Recipient.STREAM,
     )
 
+SubInfo = TypedDict('SubInfo', {
+    'sub': Subscription,
+    'stream': Stream,
+})
+
+def get_bulk_stream_subscriber_info(user_profiles, stream_dict):
+    # type: (List[UserProfile], Dict[int, Stream]) -> Dict[int, List[Tuple[Subscription, Stream]]]
+
+    stream_ids = stream_dict.keys()
+
+    result = {
+        user_profile.id: []
+        for user_profile in user_profiles
+    }  # type: Dict[int, List[Tuple[Subscription, Stream]]]
+
+    subs = Subscription.objects.filter(
+        user_profile__in=user_profiles,
+        recipient__type=Recipient.STREAM,
+        recipient__type_id__in=stream_ids,
+        active=True,
+    ).select_related('user_profile', 'recipient')
+
+    for sub in subs:
+        user_profile_id = sub.user_profile_id
+        stream_id = sub.recipient.type_id
+        stream = stream_dict[stream_id]
+        result[user_profile_id].append((sub, stream))
+
+    return result
+
 def num_subscribers_for_stream_id(stream_id):
     # type: (int) -> int
     return get_active_subscriptions_for_stream_id(stream_id).filter(