Fixed #21829 -- Added default AppConfigs.

Thanks Russell for the report, Marc for the initial patch, Carl for the
final review, and everyone who contributed to the design discussion.

tests/apps/default_config_app/__init__.py

@@ -0,0 +1 @@
+default_app_config = 'apps.default_config_app.apps.CustomConfig'

tests/apps/default_config_app/apps.py

@@ -0,0 +1,5 @@
+from django.apps import AppConfig
+
+
+class CustomConfig(AppConfig):
+    name = 'apps.default_config_app'

tests/apps/tests.py

@@ -7,6 +7,7 @@ from django.db import models
 from django.test import TestCase, override_settings
 from django.utils import six
 
+from .default_config_app.apps import CustomConfig
 from .models import TotallyNormal, SoAlternative, new_apps
 
 
@@ -82,6 +83,11 @@ class AppsTests(TestCase):
             with self.settings(INSTALLED_APPS=['apps.apps.NoSuchConfig']):
                 pass
 
+    def test_default_app_config(self):
+        with self.settings(INSTALLED_APPS=['apps.default_config_app']):
+            config = apps.get_app_config('default_config_app')
+        self.assertIsInstance(config, CustomConfig)
+
     @override_settings(INSTALLED_APPS=SOME_INSTALLED_APPS)
     def test_get_app_configs(self):
         """