If a url_name is invalid, then log the exception instead of silently discarding that menu item.

* 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

- Remove outdated material relating to old versions of Django
- State that Oscar assumes that the email field is unique
- Update a number of links to Django documentation.
- Minor fixes to other bits of documentation.

Django 1.9 introduced the AUTH_PASSWORD_VALIDATORS setting which allows for
more flexible password validation. Switch to using that when it is available,
with a fallback to Oscar's previous behaviour in Django 1.8 or when this
setting is empty.

This was removed from the parent class way back in Django 1.4, and was
not being used at all here. It existed only for backwards compatibility.

- Drop support for Django 1.9 (EOL)
- Drop support for Python 3.6 with Django 1.8 to 1.10

Upon implementing previous commit, my changes weren't picked up because
Oscar served the SimpleAddToBasketForm. If a user wants to implement
changes in both forms (which is more likely than it is not), the
previous structure meant they had to be duplicated or mixed in. Now, for
most changes, SimpleAddToBasketForm can be ignored.

The templatetag was actually never used in Oscar. See
`django-rangepaginator` for an alternative.

Deprecated in 1.4

This allows for customizing product attributes by using the common
get_class() method.