- 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

Since Django 1.11 dicts should be passed to template rendering functions

The idea of splitting integration from unittests is good in theory
but leads to a lot of mental overhead. Besides whenever a tests
interacts with a database it isn't a unittest anyway.

Better to use filenames which are commonly used in the python word then
custom ones.

It's such a common error, we should try and give a friendly error
message when it occurs.

Closes #1603.

The config was picking the wrong directory so the custom test templates
were not used during tests.

Notes

- Fix missing i18n load tags
- Remove mixed tab/space indentation
- Add two settings tests

This avoids conflicts with other libraries that use the same trick for
extending templates.

Also split them into unit and functional tests.