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

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.

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

See #1674.

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.

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.

The template expects a pagination object which was not being created due to missing paginate_by property.

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`.

Use django.core.signing instead of our own md5 hash.

Fixes #2667.

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

The blocking dependency (Haystack) now supports Django 2.

during development of large ecommerce site we ran into an exception
when accessing the list of offers and ranges:

AttributeError 'NoneType' object has no attribute 'paginator'
django_rangepaginator/templatetags/rangepaginator.py in paginate at line 12

This PR fixes all cases I could find where this happens, but chances are
some views need the same fix.

- split out the queryset filters into separate methods for easy overloading
  in subclasses, i.e. have the keyword search in product titles as well.

- cleanup the get_date_from_to_queryset method a little to make it more dry.

- expand test-coverage for the affected code.

Refs #2567 - the changes made there fail if the stock record
belongs to a child product.

There is a subtle change in behaviour here. Null is converted to ''
only when reading the field value from the database. Previously
this was done in the to_python method, but this causes problems
with uniqueness validation.