Prefix user accounts we create

- Helps protect against users named 'root'
- Makes it clearer that you should not rely on these users
  for general PAM work, because they are prefixed.

Fixes #9

tljh/configurer.py

@@ -49,6 +49,7 @@ def apply_yaml_config(path, c):
     update_userlists(c, tljh_config)
     update_limits(c, tljh_config)
     update_user_environment(c, tljh_config)
+    update_user_account_config(c, tljh_config)
 
 
 def update_auth(c, config):
@@ -102,6 +103,10 @@ def update_user_environment(c, config):
         c.Spawner.default_url = '/nteract'
 
 
+def update_user_account_config(c, config):
+    c.SystemdSpawner.username_template = 'jupyter-{USERNAME}'
+
+
 def _merge_dictionaries(a, b, path=None, update=True):
     """
     Merge two dictionaries recursively.