Django 1.6 happily allows an application namespaec (app_name) to be

specified for any URLs, but newer 1.x versions only allow this if an

instance namespace (namespace) is being provided. Django 2.x doesn't

allow the parameter at all, instead requiring a whole different process

for specifying the app namespace.
This change removes the application namespace entirely for integration

URLs. We don't use it to resolve URLs ourselves, and callers can always

provide their own namespace, so there's not much point in keeping this

around.

Djblets and Review Board unit tests pass on Django 1.6 and 1.11.