♻️[#261] change Onderwerpobject inline to use klantcontact

[Coperh]

Fixes #261

[Coperh]

I'd say the klantcontact relation is more correct since its a required field.
Maybe keep was_klantcontact but make it read-only?

It does duplicate inline if both klantcontact and was_klantcontact are set to the same klantcontact.

[Coperh]

Also I do not think you cam filter on klantcontact on the onderwerpobject endpoint

[stevenbal]

@Coperh I think the change you made is correct. I don't think it will happen in practice that both klantcontact and was_klantcontact point to the same KlantContact instance