Issue: https://github.com/django-oscar/django-oscar/issues/3162

* Checking of `offer.benefit.max_affected_items` removed from the
`available` method of `LineOfferConsumer` class - available items should
not be limited by related benefit (it should be considered when applying
the offer).
* Improved `apply` method of `PercentageDiscountBenefit` - affected
items must include "quantities with discount applied".
* `is_satisfied` method of `CountCondition` updated to pass `offer`
instead of `None` to `line.quantity_without_offer_discount` - this
change is required to fix the initial issue.
* Updated `test_available_with_offer` test - initially this test was
added to check the lines that we removed from the `available` method of
`LineOfferConsumer` class (See fda27a183d/).
* Added tests to confirm that the initial issue is fixed.

Non-exclusive offers that only allow combination with specific other offers

* enforce max_affected_items during offer application

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

* improve test coverage

* flesh-out initial opaque data-model

refactors the basket line consumption into a facade object.

the goals is accomodating combitions of various offers on the same basket-lines,
while remaining backward compatible with the current single offer per line
policy.

* add docstrings

* add exclusive flag to conditionaloffer model and continue refactoring

* test new offer consumption on basket view

* cleanups

* fix exclusivity

* code style

* add cast to pacify tests

* spelling

* add some documentation to the release notes

* move documentation to new 1.6 release docs