Fixed #24075 -- Used post-migration models in contrib apps receivers.

Thanks Markus and Tim for the review.

tests/sites_tests/tests.py

@@ -1,6 +1,7 @@
 from __future__ import unicode_literals
 
 from django.apps import apps
+from django.apps.registry import Apps
 from django.conf import settings
 from django.contrib.sites import models
 from django.contrib.sites.management import create_default_site
@@ -293,6 +294,14 @@ class CreateDefaultSiteTests(TestCase):
         create_default_site(self.app_config, verbosity=0)
         self.assertEqual(Site.objects.get().pk, 1)
 
+    def test_unavailable_site_model(self):
+        """
+        #24075 - A Site shouldn't be created if the model isn't available.
+        """
+        apps = Apps()
+        create_default_site(self.app_config, verbosity=0, apps=apps)
+        self.assertFalse(Site.objects.exists())
+
 
 class MiddlewareTest(TestCase):