- Move Oscar app configs into Django app config classes
- Rename Oscar/Django apps' app config module
- Remove "oscar.get_core_apps()" and its app overriding
- Remove "oscar.OSCAR_CORE_APPS"
- Port app forking code to work with Django app registry
- Port class loading code to use app configs to get app properties

Previously, validation did not pass when current locale is "en-gb" and URL is "/de/catalogue/" and visa versa due to default Django URL resolvers behaviour  - they do not take into account all locales, only current.

This fixes a DeprecationWarning

This has been around since forever, and removing it is overdue. I took
the opportunity to slightly improve the urls.py files.

The function essentially does two things:
1. it applies a prefix - the first argument - to the given urls. In no
   instance a prefix was given
2. it calls url() for simple tuple or list elements in the given iterable.
   Cases where such a tuple or list was passed to patterns have been
   replaced by direct calls to url()

2 allowed for both calls to url() and simple tuples to be listed in
get_urls() and similar places. This allowed for unnecessary variation.
Always calling url() makes the url lists look less noisy and more uniform,
allowing the reader to focus on the actual differences of the urls instead
of superfluities.

django.conf.urls.patterns will also undergo deprecation starting from
Django 1.8. This commit anticipates that change.

https://docs.djangoproject.com/en/dev/releases/1.8/#django-conf-urls-patterns

This required:
* Moving Django's i18n urls into the main urls.py and outside of the
  scope of Oscar's main application class, because they need to live
  outside of the scope of i18n_patterns
* Add a language-code stripped version of the current URL to the
  template context to get the language switcher working.

I also updated the language switcher code to mimic what is currently
given as an example in Django's docs.

Fixes #797

Distinguish it from the other folders which each define a category of
test.

product image app