Ensure __webpack_public_path__ for nbextension

[vidartf]

C.f. #37

[vidartf]

@jasongrout Does this look OK?

[maartenbreddels]

I thought we wanted to get rid of it, can you explain why we would want to add it?

[vidartf]

You want to get rid of it for jupyterlab extensions, but you might need it for classic notebook extensions.

[vidartf]

The docstring describes it more fully. However, the concern mentioned there doesn't matter for lab, where everything is bundled (including static assets).

[maartenbreddels]

In my recollection (might be a gitter chat) there was no use for this (any more?)

[jasongrout]

There could be a use for it in the classic notebook. I don't know anyone actually using it.

[jasongrout]

I think this change looks like it probably is okay, but I haven't tested it.

[SylvainCorlay]

We may want to check this on ipyleaflet wrt marker images. I think that we don't need it anymore though.

[vidartf]

The line in ipyleaflet is here: https://github.com/jupyter-widgets/ipyleaflet/blob/658b21564dffaa85941bf73d1875121786a6ff92/js/src/notebook.js

[SylvainCorlay]

It seems to be matching perfectly!

[maartenbreddels]

I know what the reason was I removed it in ipyvolume (and now also in ipywebrtc)
I include the index.js for the Jupyter Lab extension, and that causes all widgets to fail without any error msg. This caused me quite some time to figure out again, so I though I should write it down at least somewhere. Sigh.. but, ipywebrtc in Jupyter Lab is working!