relies on CHP's host-based routing (a feature I didn't add!)
requires wildcard DNS and wildcard SSL for a proper setup
still lots to workout and cleanup in terms of cookies and where to use host, domain, path, but it works locally.
moves `spawn_pending` flag to only around start, not the HTTP wait.
Some Spawners may not know how to poll until start has finished (DockerSpawner).
Let's not require that they do.
Usernames that have an `@'-separated domain component
break JupyterHub when the server expects to see query
strings that contain an `@', when browsers and other
clients send `%40'.
If Spawner.options_form is specified, a form providing input controls is shown to the user prior to launch.
Spawners access the result via the `self.user_options` dict.
The default spawners offer no form.
