get_available_shipping_methods must always return an iterable.

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.

The ability to include(admin.site.urls) was removed in Django 2.

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.

Support for the old-style attributes was dropped in django-extra-views 0.11.

Also pin the version as the package does not use semantic versioning.

Fixes #2688.

Revert changes in #1913 and #2294 that required a stock record to have price_excl_tax set in order to report the product as available.

Add separate checks to the basket form/add logic that checks at the time of
adding a product to the basket whether a price exists, and report an error if it doesn't.

Fixes #2664.

Also forward-port 1.5.3 release notes and clean up release notes for 1.6

This can be enabled by specifying a `OSCAR_DEPRECATED_ORDER_VERIFY_KEY` setting, which is not set by default.

This allows projects to continue validating old order verification hashes while still changing the `SECRET_KEY`.

Add support for managing large amounts of vouchers grouped in sets.

Update other deprecation notices to use RemovedInDjango20.

It is installed separately in transifex.sh for travis, and not required for developing on Oscar.