jupyter_lab: include base_url in proxy_url

[quantumsteve]

This adds base_url to the jupyterlab proxy_url. By default, base_url = "/" which provides the same behavior. If base_url is different, then the proxy_url should change to reflect this.

to test:

One can recreate the same issue with just the julia-notebook container. Run it interactively to skip the startup script

docker run -p 8888:8888 -it quay.io/jupyter/julia-notebook:python-3.12 /bin/bash

start jupyter-lab and modify ServerApp.base_url

jupyter lab --no-browser --ServerApp.base_url="ipython"

open the link in your brower, run the commands in the image and get a websocker error

With this branch the websocket error disappears and pan/zoom works in the scatter plot.