Fixed #22699 -- Configure default settings in some management commands.

This makes it possible to run django.setup() in management commands that
don't need a settings module. In addition it simplifies error handling.

Thanks Claude for the review.
Fixed #21630 -- Simplified management command discovery

Thanks Aymeric Augustin for the report end the review.
Removed an old compatibility shim added in 1.0
Fix a regression introduced by f011b90363.

`django.get_version` was actually used by the module.
Argument to `reversed` must be a sequence on Py3.
Fixed #22601 -- Added a note about model inheritance.

Thanks semenov for the report.
Fixed spelling in docs.