Remove outdated settings and fix test runner warning with Django 1.7

ADMINS and MANAGERS aren't expected by Oscar, so we can safely
remove them. ADMINS and MANAGERS is kept for the sandbox as the hosted
sandbox (latest.oscarcommerce.com) uses the same settings and David
wouldn't get error messages otherwise.

Django 1.7 test runner changes are explained here:
https://github.com/django/django/blob/214d1e1b0f1574ecab856f0096e409109efa9523/django/core/checks/compatibility/django_1_6_0.py#L17
tl;dr: Setting it explicitly stops an annoying warning.
Explicitly set app_label on all models.

Required for Django 1.7 when overriding apps
Conflicts:
	oscar/apps/partner/abstract_models.py
Use altered StockRecord model in tests

The test suite brings it's own stock record model for testing. Django
1.7 requires that we only import models from apps in INSTALLED_APPS, so
we need to use it to avoid clashes.
Only declare Oscar models if they haven't been registered before

This is necessary, as Django 1.7 enforces that a model can only be
registered once. Previously, declaring a model a second time would lead
to the second declaration being silently ignored.

The changes have been automated by this gist:
https://gist.github.com/maikhoepfel/05e047e0205f576da1a6
Remove logic special-casing for < Django 1.5

Support for Django 1.4 and below has been removed, so there's no need
for checking for it anymore.
Set DateTimePickerInput.format on Django 1.7+

Conflicts:
	oscar/forms/widgets.py
Add AppConfigs for dashboard applications

The main benefit of this is that we relabel dashboard applications to a
format of "<original_app>_dashboard" and thus avoiding name clashes.

We should nonetheless consider moving them into the original app's
namespace, as suggested in #1383.
Skip South migration tests in Django 1.7

South isn't supported in Django 1.7 and upwards, so we can only skip
them.
wip loading
Fix AutoSlugFieldTest.test_17_migration test
Tidy receivers phase one