This removes the need to load django.test when not testing.

Refs #23641.

Thanks Collin Anderson for the review.

d562527a rename the db to using in the wrong signal.

Thanks Maik Hoepfel for the report.

Thanks Rudolph for the report.

All signals should now be registered in AppConfig.ready().

Thanks un33k for the report.

Fixed #22028 -- Documented that database signals aren't emitted for apps that laack a models module.

Thanks un33k for the suggestion and donjpacheco for the draft patch.

Thanks Jannis and Marc for the feedback.

Fixed #21717.

De-aliased pre/post_syncdb to pre/post_migrate to increase
backwards-compatibility.

* override_settings may now be imported from django.test
* removed Approximate from django.test
* updated documentation for things importable from django.test

Thanks akaariai for the suggestion.

Older versions of uWSGI and Sentry's middleware do not adhere to
the WSGI spec and cause the `request_finished` signal to never
fire. Added notes to the appropriate places in the docs.

Fixed #20537.

Added -n to sphinx builds to catch issues going forward.

The auth doc was a single page which had grown unwieldy.
This refactor split and grouped the content into sub-topics.
Additional corrections and cleanups were made along the way.

Thanks Tim Graham for the review and suggestions.