Use the Hub API to spawn users

[minrk]

TODO list (mostly for min):

We should create a NullAuthenticator and use the Hub API to spawn users with tokens, rather than tmpauthenticator

• demonstrate that this works (set cookie on singleuser when authenticated with ?token=... jupyterhub#1383)
• create NullAuthenticator, which doesn't allow logging in at all
• use the Hub API to spawn servers and retrieve tokens instead of redirect to /tmplogin
• support deleting users in cull_idle_servers, rather than just stopping servers add —cull-users to cull_idle_servers jupyterhub#1380
• take a pass on errors on the JupyterHub side (handle lots of errors #91 covers lots of errors on the binderhub side, but not failure to run after the fact)
• add token for binder to the Hub via (non-managed) services config
• validate images by running jupyterhub-singleuser --version after build

[yuvipanda]

Most of these have just been deployed!

[minrk]

#135 / #142 deals with failure to launch by including launch errors in the event stream. The image validation is also less significant, since now we only assume that the image is a web server, and not jupyterhub-singleuser.

[yuvipanda]

We do this now!