Sets up a login screen using tuigreet. From there the user is thrown
into a shell. Other units/services (usually, the user service `river`)
take care to start an actual wayland session for the user.
Creates separate snapper services for root and home directories. Does
_not_ yet set up any btrfs mounts or ensure that they are mounted at the
locations.
For now, putting the associated files simply within the books/ directory
as well is the simplest solution. It also removes some clutter from the
root directory. If we refactor later, this will be changed overall
anyway.
