[newforms-admin] Delete view doesn't link to related objects

[Daniel Roseman <daniel@…>]

The get_deleted_objects function in django.contrib.admin.util lists the related objects that will be affected by a delete operation. It contains code to link each item to its actual admin page. However, this is never called because in order to check if the model has an admin page, it checks has_admin = related.class in admin_site._registry - but related.class is always <class 'django.db.models.related.RelatedObject'>, which naturally does not match anything in _registry. This needs to be changed to related.model, which does work.

Patch includes this change, plus big changes to the admin_views regression test and fixture to set up a related model to test this against.

[Daniel Roseman <daniel@…>]

Patch and updated tests/fixtures