support embedding in iframes, perhaps via custom/custom.js

[ajbouh]

Description

Based on https://jupyter-notebook.readthedocs.io/en/stable/public_server.html#embedding-the-notebook-in-another-website

One must add a custom/custom.js file to $JUPYTER_CONFIG_DIR:

define(['base/js/namespace'], function (Jupyter) {
  Jupyter._target = '_self';
});

Based on my initial experiments, it doesn't seem like this file is being included by Jupyverse into the served frontend. Is the expected? Intended?

The core thing I'm trying to do is use my jupyverse instance from within an iframe. Trying to use the above approach may be a dead-end. I'd be just as happy to do something else, like set a proper "url prefix" to use when opening links/new tabs from places like /retro/tree.

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