Custom About Dialog

[jtpio]

Problem

As more people get to try JupyterLite by going to the demo link directly: https://jupyterlite.vercel.app/

We should provide a way for them to reach the repo, for example by swapping the about dialog for a custom one:

Proposed Solution

The about plugin was made more modular upstream in jupyterlab/jupyterlab#10089.

So this will be easier to implement after updating to the 3.1 alpha packages.

[jtpio]

The about dialog could also be the same for both lab and retro.