Add some controllers + tests

This matches what is used in `modules/core/www/frontpage_config.php` for the classic UI.

This is now combined with the "SAML 2.0 IdP" enabled/disabled flag
which was a bit lonely before, as it's the only flag of its type.

A new one is in the making but it's completely different.

Hook 'metadata_hosted' and 'frontpage' are no longer called so do
not register hooks for them. Update the docs. Remove useless call
to configpage hook on admin module Test page (likely a copy-paste
error).