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/jupyterhub_config.py

@@ -1,26 +1,29 @@
 """
 JupyterHub config for the littlest jupyterhub.
-
-This is run on startup & restarts. This file has the following
-responsibilities:
-
-1. Set up & maintain user conda environment
-2. Configure JupyterHub from YAML file
-
-This code will run as an unprivileged user, but with unlimited
-sudo access. Code here can block, since it all runs before JupyterHub
-starts.
 """
-from tljh import conda
+from escapism import escape
 import os
+from systemdspawner import SystemdSpawner
+from tljh import user
 
-INSTALL_PREFIX = os.environ.get('TLJH_INSTALL_PREFIX', '/opt/tljh')
+INSTALL_PREFIX = os.environ.get('TLJH_INSTALL_PREFIX')
 USER_ENV_PREFIX = os.path.join(INSTALL_PREFIX, 'user')
 
-c.JupyterHub.spawner_class = 'systemdspawner.SystemdSpawner'
+class CustomSpawner(SystemdSpawner):
+    def start(self):
+        """
+        Perform system user activities before starting server
+        """
+        # FIXME: Move this elsewhere? Into the Authenticator?
+        user.ensure_user(self.user.name)
+        user.ensure_user_group(self.user.name, 'jupyterhub-users')
+        if self.user.admin:
+            user.ensure_user_group(self.user.name, 'jupyterhub-admins')
+        return super().start()
+
+
+c.JupyterHub.spawner_class = CustomSpawner
 c.JupyterHub.authenticator_class = 'dummyauthenticator.DummyAuthenticator'
 
 c.SystemdSpawner.extra_paths = [os.path.join(USER_ENV_PREFIX, 'bin')]
 c.SystemdSpawner.use_sudo = True
-
-c.SystemdSpawner.dynamic_users = True