bulk_create: Fix buggy logic for generating recipients_by_email.

This buggy logic from e1686f427c had
broken do-destroy-rebuild-test-database.

Now that we're not just trying to add the Recipient objects for every
user on the system here to profiles_by_id, we also shouldn't be
processing every Recipeint object on the system.  The fix is simple:
because of the patch we got merged into Django upstream,
recipients_to_create actually has the object IDs added to the
Recipient objects passed into Recipient.objects.bulk_create.

This was missed in manual testing, since it only broke `populate_db
--test-suite`.

zerver/lib/bulk_create.py

@@ -48,7 +48,7 @@ def bulk_create_users(realm: Realm,
     Recipient.objects.bulk_create(recipients_to_create)
 
     recipients_by_email = {}  # type: Dict[Text, Recipient]
-    for recipient in Recipient.objects.filter(type=Recipient.PERSONAL):
+    for recipient in recipients_to_create:
         recipients_by_email[profiles_by_id[recipient.type_id].email] = recipient
 
     subscriptions_to_create = []  # type: List[Subscription]