Make NoShippingRequired available by default.

When all product classes have shipping_required=False the checkout logic
automatically returns the shipping method NoShippingRequired. That shipping
method is not available in the default Repository so the checkout will
crash.

Adding NoShippingRequired allows the checkout to work in that cases without
overriding the Repository class.
Merge branch 'origin/fix/shipping_method_failure' into machtfit

* branch origin/fix/shipping_method_failure:
  Allow shipping_address being 'None' for an order
Merge branch 'add-no-shipping-required-to-default-repository' into machtfit

* branch add-no-shipping-required-to-default-repository:
  Make NoShippingRequired available by default.

Issue #999 Checkout broken if all products don't require shipping
https://github.com/tangentlabs/django-oscar/issues/999
https://groups.google.com/forum/#!msg/django-oscar/nC3rJV54oZQ/Ms_zcTd54bAJ
PR #1: https://github.com/tangentlabs/django-oscar/pull/956
PR #2: hab ich keinen erstellt weil die Diskussion noch ongoing ist
für PR #1 gibt es eine Alternative: https://github.com/tangentlabs/django-oscar/pull/910
Merge branch 'fix-partner-migrations' into machtfit

* branch fix-partner-migrations:
  Update migration to include PartnerAddress model.

PR: https://github.com/tangentlabs/django-oscar/pull/970
Merge branch 'dashboard-partner-allow-not-only-staff' into machtfit

* branch dashboard-partner-allow-not-only-staff:
  Pre-select partner if only one is available and remove empty option
  Require at least one stock record for partner dashboard users
  Restrict stock record editing to the current user's partners if not is_staff
  Allow non-staff users to be associated with a partner.
  Fix whitespace.

Shortcomings in permission based dashboard
My PR: https://github.com/tangentlabs/django-oscar/pull/968
der enthält aber mehrere Fixes, bei denen nicht klar ist, wie die Problematik in Oscar
implementiert werden soll. Split in einen PR und ein Issue:
PR: https://github.com/tangentlabs/django-oscar/pull/1001
    Enthält
Merge branch 'template-customization' into machtfit

* branch template-customization:
  Add blocks to make template customization easier

PR: https://github.com/tangentlabs/django-oscar/pull/1004