With firstuseauthenticator, users need to be manually created
in the admin console before they can log in. This makes for a
far more secure default experience.
- 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
