Fix handling of count conditions in `MultibuyDiscountBenefit`

* enforce max_affected_items during offer application

also restrict the usage of offer consumption to line.consume()

* improve test coverage

- 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.

This order offers the simplest API for common needs - as evidenced by the
fact that the apply_offers crook could be removed from two test files,
saving on imports as well.

This already worked only within site offers by way of default model
ordering. Site offers are probably the most common offers, however the
ordering of all offers has to be controlled by the applicator.

Conflicts:
	tests/integration/offer/applicator_tests.py

Cherry-picked from #1414.

Hmmm, this turned out to be a monster commit.  This change allows the
basket to be able to correctly calculate prices including tax.

It also requires a whole load of test changes since all baskets now
require a strategy instance to be assigned.

This allows updating code in both places, if necessary. Getting Django
1.5 support would be painful otherwise. This also removes the circular
dependency of django-oscar-testsupport on Oscar.

The requirements were merged. The imports were updated
accordingly. Unused imports in the touched files were removed. No
further changes.

This paves the way for custom benefits to return successful applications
that don't lead to a discount.

This might be futile.

* Includes a migration for the proxy_class field
* Dashboard templates needed adjusting

Fixes #386

Renaming column for clarity now that we have alternative fields for
tracking consumption counts

Fixes #376

Added new field to offer that allows their max num applications to be
controlled.  Dashboard adjusted to support this too.