Laurence de Bruxelles

Fixing this in #328.

This would be useful for the Learn to Code repo, which has an issue because Redacarpet doesn't support Markdown in HTML in the same way CommonMark does: https://github.com/alphagov/learn-to-code/issues/56

> If I understand what this is doing correctly, then I think there's a neater way to do this that doesn't need us to monkey patch the `asset_path` function. >...

> I'm guessing there's no easy way to test this, given the functionality we're interested in is really part of Sprockets? > > Otherwise I'm happy with this approach, bar...