Feature/11 producttype eigenschappen

[Floris272]

Changes

• Adds Eigenschap & ExterneCode models with fk to ProductType.
• Both models are key value pairs and are created within the ProductType post request.
• The same filter on the ProductTypeViewSet has been added to both models:
  • ?eigenschap=[key:value]
  • ?externe_code=[key:value]
  • The format is currently formatted using regex and i also added regex to the model fields, not sure if this is the best way.