Fix html_embed exporter

[juhasch]

Fixes issue #1041:
Correctly embed images already embedded in the notebook itself.

[gabyx]

was the path already in the metadata?
I thought there is not path variable in the resource metadata :-)
if so, this is a cool fix :-) fixes my export button to

[juhasch]

This works at when calling nbconvert from command line.
I don't have notebook 5.1 running, so this might not work with your extension. However, I would assume this is a bug in the notebook then.

[gabyx]

ah, ok, hm..., so this merge will
make my extension not working before the notebook bug is not resolved...
maybe I have some time

[juhasch]

Hmm, looks like the notebook calls from_notebook_node() and does not provide the path. In my eyes it should. This means I need to add a check in the exporter, and your problem with the extension is still not solved.

[gabyx]

Created a pull request to fix that:
jupyter/notebook#2753