Add a new fixture: admin_user

This factors the admin user out of the `admin_client` fixture and
provides it separately.

TBD: it also adds `django_user_model` and `django_username_field`
fixtures, which I have factored out when writing a test for the
`admin_user` fixture.
Useful? Should they be renamed to not have the `django_` prefix? (a more
explicit name is better probably)?