feat: introduce an ImageContent component in editor

[eric-burel]

This PR introduces an ImageContent component for the editor, for binary images.

Design choices:

• only binary images are supported, with format you typically expect on the web
• svg is still treated as text (you might want a tutorial about programming with SVG)
• other binary image format are not supported (which I think is ok for most use cases)

Problems left:

• somehow the editor displays code from another file alongside the image
• not sure if shoving images in the first demo lesson is good for testing

Closes #309