* 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

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.