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)

Subsequent views of the page (for the same order) should not trigger an event.

Fixes #2413.

Fixes #2277.

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.