use JupyterHub 0.8 features, if available
use JupyterHub 0.8 features, if available

Specifically:

- .login_user()
- .get_next_url()

Reduces duplication from JupyterHub implementations

jupyter/oauthenticator,minrk/oauthenticator,NickolausDS/oauthenticator,jupyterhub/oauthenticator,enolfc/oauthenticator,jupyter/oauthenticator,maltevogl/oauthenticator