Integrate with Dask JupyterLab extension to create clusters

[JoshKarpel]

https://github.com/dask/dask-labextension#configuration-of-dask-cluster-management

Looks like we "just" (famous last words) need to ship some more config goop in our config file.

[JoshKarpel]

Unfortunately, it looks like we can't "merge" a new cluster definition into the labextension config, because you can only have one cluster defined at a time (a consequence of the way they've structured their YAML). They ship a commented-out ~/.config/dask/labextension.yaml that you're supposed to edit, and I've confirmed that it can "find" our CHTCCluster if configured properly. I think the most we can do is provide config snippets in the forthcoming configuration guide #15 . I've added a note on that issue.