Load user lists & auth config from a YAML file

- Load config only once at startup.
  A lot of jupyterhub config (like user lists) take effect only at
  startup, so live reload is not super useful. It will make the
  software more complex, so let's not do it.
- Add pyyaml as a dependency of tljh.
- Remove escapism dependency since it is not actually used

tljh/jupyterhub_config.py

@@ -1,10 +1,9 @@
 """
 JupyterHub config for the littlest jupyterhub.
 """
-from escapism import escape
 import os
 from systemdspawner import SystemdSpawner
-from tljh import user
+from tljh import user, configurer
 
 INSTALL_PREFIX = os.environ.get('TLJH_INSTALL_PREFIX')
 USER_ENV_PREFIX = os.path.join(INSTALL_PREFIX, 'user')
@@ -23,9 +22,9 @@ class CustomSpawner(SystemdSpawner):
             user.remove_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
+
+configurer.apply_yaml_config('/etc/jupyterhub/jupyterhub.yaml', c)