The initial check if the voucher is available was done before creating
the order. This change adds a validation step during the order creation
within an atomic() block.

See #2101

Most real-world instances will use postgresql instead of sqlite so it's
better to use postgres for the tests. This also allows us to easily add
tests for concurrency issues which is not supported by sqlite

The pk kwarg on views is a string type. The message format string was expecting a numeric value. This updates that format string to expect a string value, which will always display correctly.

As in `test_model.py` -> `test_models.py`

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.

Latter one is deprecated

This ensures that migrations are not required when the OSCAR_MODERATE_REVIEWS setting is changed.

This also updates the pytest dependencies

Since there is no way to override SlugField and AutoSlugField in existing Oscar models, it was decided to pass `allow_unicode` parameter if this setting enable. This allows to achieve backwards compatibility with Django versions under 1.9, when this parameter was not supported yet. If parameter specified in the field expliticly - we do not override it, which allow to control slug fields individually in a normal way. For the reasons, explained above, standard SlugField was replaced with a custom one with the single slight change - control over `allow_unicode` parameter by the mentioned setting.

basket.Line.quantity is a positive integer field, but the quantity sumation in
basket.Basket.add_product made it possible to try and save negative values. This
makes add_product never go below 0-quantity, thereby preventing a db exception
from being thrown. Also adds a test for the described behavior.

The Repository.apply_shipping_offer() method returns the shipping method
object when there is no additional discount for an order. This however
broke with the WeightBased, because it's implementation does not work.
self.offer is never assigned, nor useful (a copy-paste error?).

Instead, it should work like the non-model `methods.Base` that also
states the method does not have a default shipping discount.

While the backend stores the information as floats, the calculations
don't have to do be affected by this. For example:

0.3 + 0.3 + 0.3 == 0.8999999999

Using decimals improves the calculation, and avoid falling into the
wrong weight band.

 - Set Meta.app_label on models created in the tests
 - Use logging.NullHandler instead of django.utils.log.NullHandler
 - Use oscar.core.loading.get_model() instead of django's get_model
 - Remove template tags '{% load url from future %}'