Use classic unix users rather than systemd dynamic users

Dynamic Users are neat and probably very useful for a tmpnb
style situation. However, for regular use they have the following
problems:

1. Can't set ProtectHome=no, so you can never apt install or
   similar from inside admin accounts.
2. Dynamic uid / gid makes it hard to write sudo rules. We want
   admin users to have sudo.
3. Persistent uids / gids are very useful for ad-hoc ACLs between
   users. gid sharing isn't the most flexible sharing mechanism,
   but it is well known & quite useful.
4. /etc/skel is pretty useful!

tljh/systemd-units/jupyterhub.service

@@ -7,6 +7,7 @@ Wants=network-online.target
 [Service]
 User=root
 Restart=always
+Environment=TLJH_INSTALL_PREFIX={install_prefix}
 StateDirectory=jupyterhub
 WorkingDirectory=/var/lib/jupyterhub
 ExecStart={python_interpreter_path} -m jupyterhub.app -f {jupyterhub_config_path}