phone_number_fields is more correct grammatically.

Add to the dosctring for the class, and display it in the documentation.

With this change phone numbers (any qty) can be added as `ModelForm`
property `phone_numbers_fields` - dict where "keys" are names of
phone numbers fields and "values" are attributes of fields.

* `PhoneNumberMixin` clightly reworked and moved
from `oscar.views.generic` to `oscar.forms.mixins`
* Couple tests added (based on previous tests)

Fixes #2414.

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.

It seems that migrations are unicode in Django 1.11

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.

* Tidy implementation of validator
* Tests improved and moved to tests/integration (as they call resolve
  internally)