this commit removes the static assignments for the ntfy-sh user and
group. furthermore, it removes some tmpfiles.d rules which where
initially put in place by https://github.com/NixOS/nixpkgs/pull/234811.
these are however not required, as ntfy-sh will automatically create the
required files and systemd automatically handles the migration process.
A nixosTest is added to demonstrate that the migration is working
reliably.
This also fixes an issue with where systemd would sometimes not start
ntfy-sh. The tmpfiles rules in combination with impermanence caused `/
var/lib/ntfy-sh` to be a directory when it should have been a symlink.
