* feat :star: auto select existing user when an account already exists

* lint

* Added test and simplyfied code

---------

Co-authored-by: Lars van de Kerkhof <lars@permanentmarkers.nl>

- Replace use of deprecated `request.is_ajax()` with equivalent.
- Replace use of `imp.reload` with `importlib.reload`

Better to use filenames which are commonly used in the python word then
custom ones.

Future tasks is to dig into self.assertRedirectsTo wouldn't work with the test
as it stands

Adds new functional tests for ShippingAddressView, PaymentDetailsView
and ThankYouView.
It's the second part of #1645.

Notes

* Adds a custom template to be able to test some functionalities of
  PaymentDetailsView
* Adds an helper to the CheckoutMixin

* Uses factory-boy factories
* Adds helpers to the CheckoutMixin helper
* Adds plenty of functional tests
* Adds HTML IDs to some forms to be able to easily reference them
  from our tests

@maikhoepfel:
This commit is the squashed version of #1645, but it
does not contain the template changes. The unused
ShippingAddressView code was already cherry-picked across in 549decb.

I also fixed some PEP8 issues.

We have two methods doing something very similiar, so I'm removing the
one with the worse name.

Thanks go to @itbabu for spotting it in
https://github.com/django-oscar/django-oscar/pull/1645#issuecomment-71896876

The previous skip condition for shipping bypassed the shipping method
view but this is required as it sets the shipping method code.

When ordering a basket of free products, or with vouchers giving 100%
discount, we should not collect payment details.

To achieve this, I've added a check to the CheckoutSessionMixin which
calculates the order total and decides whether payment is necessary.

The modified test failed because the OrderCalculator called by
check_payment_is_necessary accessed other properties on the shipping
method. We now just mock a valid shipping method, which still achieves
the aim of ensuring there's only one.

A bit OCD, but I noticed that the checkout forms were an exception in
using a hyphen so I've cleaned up all IDs to use underscores.

We no longer use Django's test client.

- Comment the session manager better.
- Clean up the functional checkout tests.

It is now replaced with a checkout pre-condition that applies to every
checkout view. If someone's basket becomes invalid while they are in
checkout, then they will be redirected back to the basket page.