* Allow clearing default user backends in config.php

When specifying "user_backends" in config.php, a new option "default"
when set to false will prevent the default user backend to be
registered. The default one is the database backend.
This makes it possible to select exclusive user backends from apps.

* Testing app provides test user backend for alternative homes

The backend provide md5 result to getHome()

* Only md5 the user home when it's not the admin