Fixed #21712 -- Added AdminConfig with autodiscover.

A couple of points for consideration:

- `app.py` or `apps.py`? This will set a convention for everything that follows. My inclination suggests `apps.py` as it is plausible at least some apps will have multiple options. Also forms, models, views, urls etc are all plural so it fits nicer.
- Should I test this? I think it's possible using the installed apps override and resetting the admin registry, but I'm not sure if it's useful. My inclination says "test everything" though.
- Should we change the default project? (This is more work and I'm inclined to open an extra ticket for it).