[POC] getconf_admin: add the environment to the admin banner

The idea is to add a flag to the Django admin banner when this is not the production environment.

I don't add anything for "prod" not to disturb people.

We could also think of a color code.

The issue is see is that admin_base.html file that could change from version of Django to another (though unlikely).

This is just a sketch to discuss, the code was not even tried.