Have the notebook surver support HTTP basic auth for passwords

Genesis of this is my question here in github help:
jupyter/help#476

We're currently using the @jupyterlab/services to connect to the notebook server. However those services currently only support token authentication for connecting. For password support you still need to hit the login page and generate a cookie to use, so you can't use it with the services.

If the notebook server supports http basic authentication then the services can support connecting to a notebook server with a password instead of a token. This would make the services much more usable for consumers.

I've already filed an issue on the jupyterlab services side regarding this:
https://github.com/jupyterlab/jupyterlab/issues/5927

But they have indicated that the http basic auth work needs to happen on the notebook server first, so I've filed this here.

Start by reviewing the notebook server's existing token and password authentication flow, along with jupyter/help#476 and the linked JupyterLab services issue. Done means the server supports HTTP Basic Authentication with a password so @jupyterlab/services can connect without generating a login cookie or token.

Written by the indexing model from the issue text.