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.