Moved list of models inside AppConfig instances.

This commit is a refactoring with no change of functionality, according
to the following invariants:

- An app_label that was in app_configs and app_models stays in
  app_config and has its 'installed' attribute set to True.

- An app_label that was in app_models but not in app_configs is added to
  app_configs and has its 'installed' attribute set to True.

As a consequence, all the code that iterated on app_configs is modified
to check for the 'installed' attribute. Code that iterated on app_models
is rewritten in terms of app_configs.

Many tests that stored and restored the state of the app cache were
updated.

In the long term, we should reconsider the usefulness of allowing
importing models from non-installed applications. This doesn't sound
particularly useful, can be a trap in some circumstances, and causes
significant complexity in sensitive areas of Django.

tests/tablespaces/tests.py

@@ -1,7 +1,5 @@
 from __future__ import unicode_literals
 
-import copy
-
 from django.apps import app_cache
 from django.conf import settings
 from django.db import connection
@@ -28,7 +26,7 @@ class TablespacesTests(TestCase):
     def setUp(self):
         # The unmanaged models need to be removed after the test in order to
         # prevent bad interactions with the flush operation in other tests.
-        self.old_app_models = copy.deepcopy(app_cache.app_models)
+        self._old_models = app_cache.app_configs['tablespaces'].models.copy()
 
         for model in Article, Authors, Reviewers, Scientist:
             model._meta.managed = True
@@ -37,7 +35,7 @@ class TablespacesTests(TestCase):
         for model in Article, Authors, Reviewers, Scientist:
             model._meta.managed = False
 
-        app_cache.app_models = self.old_app_models
+        app_cache.app_configs['tablespaces'].models = self._old_models
         app_cache._get_models_cache = {}
 
     def assertNumContains(self, haystack, needle, count):