Auto-setup cluster for users

We'd like to use the distributed dashboard without necessarily exposing the user to `dask.distributed`, `Client`, or schedulers. How can we do that?

Presumably, we can execute a bit of code in the background for the user via jupyterlab. We would need

1. Start a Dask cluster, either one time when jupyterlab is launched, or when each notebook is opened and a kernel is started.
2. Create a client in each kernel session.

Start by investigating how JupyterLab launch and kernel startup hooks can create a Dask cluster and a client without exposing users to dask.distributed, Client, or schedulers. The work is done when the distributed dashboard can be used with a cluster started automatically and a client created for each notebook kernel session.

Written by the indexing model from the issue text.