subsonic: change NixOS home directory config

Move Subsonic state directory from `/var/subsonic` to
`/var/lib/subsonic`, since the general convention is for each
application to put its state directory there.

Also, automatically set the home directory of the `subsonic` user to the
value of `config.services.subsonic.home`, rather than setting it to a
value hardcoded in the module. This keeps the home directory of the
`subsonic` user and the state directory for the Subsonic application in
sync.