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.