* Fix that the condition form required a range when choosing a custom condition

* Implement condition type validation on model.

- This uses a simpler and more robust way to get the list of fields to
  serialise for storage in the session
- "custom_benefit" and "custom_condition" were unused, as they aren't
  fields of any of the serialised models
- Also remove unused parameter in
  "dashboard.offers.views.OfferWizardStepView._fetch_form_kwargs"

- allow creation of offers of all types
- allow creation of vouchers with multiple offers
- allow creation of voucher sets of all usages, and with multiple offers

When looking up a URL for a forked app, the previous mechanism fetched the top level AppConfig which allows every user.
The new mechanism creates a dict of dashboard URLs/configs and does a lookup to find a DashboardConfig.

This will also raise NoReverseMatch if the URL does not exist or is not a dashboard URL.

fix https://github.com/django-oscar/django-oscar/issues/3080

The template expects a pagination object which was not being created due to missing paginate_by property.

Add support for managing large amounts of vouchers grouped in sets.

during development of large ecommerce site we ran into an exception
when accessing the list of offers and ranges:

AttributeError 'NoneType' object has no attribute 'paginator'
django_rangepaginator/templatetags/rangepaginator.py in paginate at line 12

This PR fixes all cases I could find where this happens, but chances are
some views need the same fix.

- split out the queryset filters into separate methods for easy overloading
  in subclasses, i.e. have the keyword search in product titles as well.

- cleanup the get_date_from_to_queryset method a little to make it more dry.

- expand test-coverage for the affected code.

If a url_name is invalid, then log the exception instead of silently discarding that menu item.

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.

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

Replace 'assert' with 'assert*' method.

Convert tests.unit.forms.widget_tests to use TestCase class.

Convert bankcard tests to use TestCase.

Convert tests.unit.logging_tests to use TestCase.

The test belongs to the other functional dashboard order tests

Update search filter strings for i18n, including translator comments.

* It can now be specified in OSCAR_DASHBOARD_DEFAULT_ACCESS_FUNCTION
* nav.py and menu.py were split up so that Node can be dynamically
  loaded and hence overridden

I recently needed to alter the access function and realised it's not as
easy as I thought.