WIP: Re-use images with mystmd embedding feature

[rossbar]

There are at least two instances where an image produced by a notebook is explicitly saved as a static image in the source tree. mystmd has an embedding feature that allows the re-use of content generated during notebook execution (including images) in the rendered pages. This PR is an attempt at integrating the embedding feature.

Marking as draft/WIP for now because it's not working quite as expected (see linked issue).