When rendering the change form for a model on an admin site, we were

using Django's admin_urlname() function to generate the namespaced URL

name to the model.
Since this is what Django itself calls, it was expected to work

regardless of the AdminSite hosting the page. However, in reality, it

just hard-codes admin as the namespace, without caring about the

actual AdminSite.
This change removes this call entirely, switching to our own URL

building logic that respects the AdminSite name.

Tested loading normal database model change forms, and extension-provided

ones.
Tested the Delete button in each case.