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

setup.py

@@ -11,6 +11,6 @@ setup(
     packages=find_packages(),
     include_package_data=True,
     install_requires=[
-        'escapism==1.*'
+        'pyyaml==4.*'
     ]
 )