Allow to disable unnecessary API registry checks via Django settings

[anentropic]

A minimal change to achieve what I needed in #1615 without changing any core functionality

I have tried this in my project and seems to work fine, I am able to do:

    path("app-a/", include("apps.a.urls", namespace="a")),
    path("app-b/", include("apps.b.urls", namespace="b")),

where apps.a.urls and apps.b.urls both look like:

api = NinjaAPI(urls_namespace="reports")
...
urlpatterns = [
    path("other", other.view, name="other"),
    path("", api.urls),
]

and then I can reverse url names like a:reports:api-view, b:reports:api-view, or non-ninja views like a:other

(I feel like the registry could probably just be removed entirely, AFAICT it's not doing anything useful)

[igoose1]

This test differs from the old one by not setting NINJA_SKIP_REGISTRY back.

[anentropic]

The pytest-django settings fixture does that automatically... accepting the fixture behaves a bit like if the test case was wrapped in a context manager

(If it wasn't getting restored then many other tests would fail)