resize partition, select keyboard layout and create new user on first boot

build an initramfs which:

 - grows the root partition and the root filesystem before it gets mounted
 - asks for the user's keyboard layout -- this prompt is layout independent as
   the number keys are used to select it
 - the keyboard layout is stored systemwide as well as in the sway config of
   the new user
 - creates a new user and adds it to the sudo group and to all groups
   that debian-installer adds new users to

There is also a systemd service which rebuilds the initramfs on first
boot and then reboots into the new system. This rebuilding results in an
initramfs which doesn't contain the first boot scripts anymore, so on
subsequent boots there will be zero delays as the first boot scripts are
not part of it anymore. The systemd service also takes care to delete
itself. This is why this service should not go into the reform-tools
package but should stay in reform-system-image repo.