Rendered markdown gets images from the wrong place

[eira-fransham]

This isn't really fixable without adding a "URL prefix" option to comrak::ComrakOptions. This is essentially how GitHub gets relative-path images from the right place in its rendered markdown. Alternatively, we can add a map method to comrak::arena_tree::Node and then map it that way (since we want images to have a prefix but not links). I'll put in a PR and then put a PR to this when that gets merged.