Use official HubAuth from JupyterHub

[jhamrick]

Fixes #520

cc @minrk -- no need for review or anything, I just wanted to let you know I've finally done this! I ended up not using the tornado mixin because my handlers aren't always authenticated with JupyterHub. But I am now using jupyterhub.services.auth.HubAuth rather than my own thing.

[minrk]

Great! Since you are the first user of this outside the single-user server itself, I'd be happy for some feedback if there's anything that should be better.

[jhamrick]

I think for the most part it works pretty well. One suggestion might be to have the default login url just be '/hub/login' (rather that including the full url at the beginning) because otherwise the ?next=... redirection doesn't work (since JupyterHub will refuse absolute redirects, only relative ones).

[minrk]

Great, thanks. I think that's something to fix on both ends - the redirect filter should recognize its own URL.