Allow override of RegisterUserMixin

The user_registered signal is not enough because we extended the
registration form with some fields that we need at user registration.
Merge branch 'partner-user-customizing' into machtfit

* branch partner-user-customizing:
  Remove now unused method Product.is_user_a_partner_user
  Extract ordering the queryset into a method
  Filter out products the the user isn't allowed to edit in one place
  Extract method that applies the dashboard product list search query

https://github.com/tangentlabs/django-oscar/pull/1059
Merge branch 'easily-add-new-formset-to-product-update-page' into machtfit

* branch easily-add-new-formset-to-product-update-page:
  Provide a hook for cross-form validation
  Make it trivially easy to add more formsets to the product update page
  Unify __init__ method signature of formset classes
  customize FormSet class after calling inlineformset_factory

https://github.com/tangentlabs/django-oscar/pull/1022

Conflicts:
	oscar/apps/dashboard/catalogue/views.py
Merge branch 'partials-product-template-block' into machtfit

* branch partials-product-template-block:
  Add template block for easier customization

https://github.com/tangentlabs/django-oscar/pull/1051
Merge branch 'passwordreset-singleobjectmixin' into machtfit

* branch passwordreset-singleobjectmixin:
  Make it easy to restrict the users you can use this form with.

https://github.com/tangentlabs/django-oscar/pull/1052
Merge branch 'bulkeditmixin-use-get-queryset' into machtfit

* branch bulkeditmixin-use-get-queryset:
  Retrieve objects from get_queryset instead of default manager.
  Don't hide base class get_queryset.

https://github.com/tangentlabs/django-oscar/pull/1065
Merge branch 'allow-registerusermixin-override' into machtfit

* branch allow-registerusermixin-override:
  Allow override of RegisterUserMixin