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.

* Improve `available` method of `LineOfferConsumer` class
* Add `quantity_available_for_offer` method to `AbstractLine` class
* Improve conditions: "Get num matches" methods updated to use `quantity_available_for_offer`
method of `Line` model.
* Improve conditions: `get_upsell_message` methods updated to show upsell message only in
the cases where `delta` > 0.
* Ensure basket upsell messages are not `None`

Also fix "is_anonymous" attribute value in WebTestCase subclass.

Use function as the return value of `get_url_decorator`. This change
needed because `get_url_decorator` method of `OscarConfigMixin` class
returns function or `None` and not string.

Co-authored-by: Basil Dubyk <samitnuk.work@gmail.com>
Co-authored-by: Alexander Gaevsky <sasha@sasha0.ru>
Co-authored-by: Joseph Wayodi <jwayodi@gmail.com>

Instead of checking for truthiness we should check for equality
(because any status code is `True`).

`override_settings` used instead of updating setting in `setUp`/`tearDown`.

Fixes #3258.

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

Currently, if a valid basket line is marked for deletion, any other
invalid basket lines on the formset prevent it from being deleted.

Fix handling of count conditions in `MultibuyDiscountBenefit`

Switch to using freezegun for tests that depend on date.