perf: Extract get_subscribed_stream_ids_for_user.

This new method prevents us from getting fat
objects from the database.

Instead, now we just get ids from the database
to build our subqueries.

Note that we could also technically eliminate
the `set(...)` wrappers in this code to have
Django make a subquery and save a round trip.
I am postponing that for another commit (since
it's still somewhat coupled to some other
complexity in `do_get_streams` that I am trying
to cut through, plus it's not the main point
of this commit.)

BEFORE:

    # old, still in use for other codepaths
    def get_stream_subscriptions_for_user(user_profile: UserProfile) -> QuerySet:
        # TODO: Change return type to QuerySet[Subscription]
        return Subscription.objects.filter(
            user_profile=user_profile,
            recipient__type=Recipient.STREAM,
        )

    user_subs = get_stream_subscriptions_for_user(user_profile).filter(
        active=True,
    ).select_related('recipient')
    recipient_check = Q(id__in=[sub.recipient.type_id for sub in user_subs])

AFTER:

    # newly added
    def get_subscribed_stream_ids_for_user(user_profile: UserProfile) -> QuerySet:
        return Subscription.objects.filter(
            user_profile_id=user_profile,
            recipient__type=Recipient.STREAM,
            active=True,
        ).values_list('recipient__type_id', flat=True)

    subscribed_stream_ids = get_subscribed_stream_ids_for_user(user_profile)
    recipient_check = Q(id__in=set(subscribed_stream_ids))

zerver/lib/stream_subscription.py

@@ -24,6 +24,13 @@ def get_active_subscriptions_for_stream_ids(stream_ids: List[int]) -> QuerySet:
         active=True
     )
 
+def get_subscribed_stream_ids_for_user(user_profile: UserProfile) -> QuerySet:
+    return Subscription.objects.filter(
+        user_profile_id=user_profile,
+        recipient__type=Recipient.STREAM,
+        active=True,
+    ).values_list('recipient__type_id', flat=True)
+
 def get_stream_subscriptions_for_user(user_profile: UserProfile) -> QuerySet:
     # TODO: Change return type to QuerySet[Subscription]
     return Subscription.objects.filter(