Update the TestDashboardIndexForAnonUser test case

Add functionality to check if a voucher is
available for a basket or not.

Also refactor catalogue option group and option tests.

- 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

This change fixes the inconsistency with range.all_products() which
didn't check for exclusions when includes_all_products is set

Every other bit of oscar expected range.includes_all_products means
that all products are included, no questions asked. The dashboard
interface even hides the option to edit/remove individual products.

The short circuiting in ConditionalOffer.products() is removed so
the range.all_products() logic is applied consistently.

Fixes #2579.

When `remaining_weight` is "0" (zero) we should not get band for it.

Django 2.1 works fine - the only issue was that more passwords
have been added to the CommonPasswordValidator which caused some tests
to fail. I've removed that validator from the test settings.

See #1674.

Pass communication event type code to OrderPlacementMixin.get_message_context()

The procedural views were deprecated in Django 1.11 and will be removed in Django 2.1.

If a phone_number field is required, and empty, then PhoneNumberMixin.clean() fails with a key error because the field does not exist in cleaned_data.

This patch fixes the clean() method to only attempt to clean a field if it exists in cleaned_data. If it doesn't then allow Django to raise a validation error for a missing required field.

Also added more comprehensive tests for the validation logic in the mixin.