Use function as the return value of `get_url_decorator`. This change
needed because `get_url_decorator` method of `OscarConfigMixin` class
returns function or `None` and not string.

- 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