DRF get_url: generates wrong urlid for ForeignKey models

DRF's relations.get_url resolves the urlid of a field by reversing the view and building the site url with the request object

The issue with this is that when the server is using an Nginx proxy for example (which transforms the request), or the SITE_URL otherwise doesn't match the request url, the url returned doesn't match the urlid of the model

In !174 (merged) we override get_url to use obj.urlid where available, but in the case of the ForeignKey it isn't available (see tests_get and tests_save referencing this issue). In get_url we don't have information available at the Model the instance represents, so using Model.get_absolute_url isn't an option with this data (obj.__dict__ during these tests is just {pk: 1})

We should possibly also open an issue with Django Rest Framework