- If start_datetime is null, then the offer has no start date.
- If end_datetime is null, then the offer never expires.

Use the ActiveOfferManager in Applicator.get_site_offers instead of
reimplementing the same logic there.

Fixes #2344.

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.

Create a factory module per app and move them under the new
oscar.test.factories module.

We now need to pass in the shipping charge explicitly as well as the
method. The way that the shipping discounts are persisted is also
adjusted.

As they are integration tests, not unit tests.