Template would fail because key does not exist, when default() is
not immediately after it. So need to split this up.

This introduces the following:

- The use of Request objects to handle request data to controllers.
- The use of Response objects to model responses that should be sent to the browser.
- The use of "controllers" that are responsible for translating a request into a response.
- The possibility to define your own URLs on each module by specifying them, together with their controllers, in a "routes.yaml" file in the root of a module.
- The new UI is completely separated from the old, so "usenewui" must be set to "true" in the configuration.
- Twigified templates are not used unless we're using the new UI, or the twig template is part of a theme.

Based on authenticate.php, login.php adds lines 6-26.
The script returns the possible authentication methods for the user, excluded admin.
When only one method is available the user is redirected to the one authentication-method login page.
Calls to right templates are updated at lines 17 and 54.

When no method is found, the user needs to login with admin credentials and check configuration.
The text shown in template is not translated.