Use get_registered_model to load User model in Django 1.7

In Django 1.7, get_user_model fails if the app cache isn't ready. As we
have to use it in many places in Oscar, this commit essentially loads
the User model like we load any other model in Oscar. This circumvents
the check for the model registry being ready.
Otherwise, there's major problems trying to avoid imports that trigger
the call to get_user_model (e.g. in forms and test factories) before the
first model is loaded (which then triggers Django's app load sequence).

oscar/core/compat.py

@@ -6,17 +6,42 @@
 from django.core.exceptions import ImproperlyConfigured
 
 
+# A setting that can be used in foreign key declarations
+AUTH_USER_MODEL = getattr(settings, 'AUTH_USER_MODEL', 'auth.User')
+# Two additional settings that are useful in South migrations when
+# specifying the user model in the FakeORM
+try:
+    AUTH_USER_APP_LABEL, AUTH_USER_MODEL_NAME = AUTH_USER_MODEL.rsplit('.', 1)
+except ValueError:
+    raise ImproperlyConfigured("AUTH_USER_MODEL must be of the form"
+                               " 'app_label.model_name'")
+
+
 def get_user_model():
     """
-    Return the User model.
+    Return the User model. Does not require the app cache to be fully initialised.
 
     This used to live in compat to support both Django 1.4's fixed User model
     and custom user models introduced thereafter.
     Support for Django 1.4 has since been dropped in Oscar, but our
     get_user_model remains because code relies on us annotating the _meta class
     with the additional fields, and other code might rely on it as well.
     """
-    model = django_get_user_model()
+
+    # As Oscar uses get_user_model all over the codebase, we need to be able to
+    # fetch the model without Django checking if the app cache is ready.
+    # Pre-Django 1.7, that check doesn't happen anyway and we can just use
+    # get_user_model. In Django 1.7, we revert to get_registered_model, which
+    # is the last remaining call to get into the model registry without
+    # Django enforcing that it's fully populated.
+    try:
+        from django.apps import apps
+    except ImportError:
+        # Django <1.7
+        model = django_get_user_model()
+    else:
+        # Django >=1.7
+        model = apps.get_registered_model(AUTH_USER_APP_LABEL, AUTH_USER_MODEL_NAME)
 
     # Test if user model has any custom fields and add attributes to the _meta
     # class
@@ -29,17 +54,6 @@ def get_user_model():
     return model
 
 
-# A setting that can be used in foreign key declarations
-AUTH_USER_MODEL = getattr(settings, 'AUTH_USER_MODEL', 'auth.User')
-# Two additional settings that are useful in South migrations when
-# specifying the user model in the FakeORM
-try:
-    AUTH_USER_APP_LABEL, AUTH_USER_MODEL_NAME = AUTH_USER_MODEL.rsplit('.', 1)
-except ValueError:
-    raise ImproperlyConfigured("AUTH_USER_MODEL must be of the form"
-                               " 'app_label.model_name'")
-
-
 def existing_user_fields(fields):
     """
     Starting with Django 1.6, the User model can be overridden  and it is no