feat: rewrite /cdn/dev urls in markdown parser on release deployment

[mcdurdin]

Currently markdown files with /cdn/dev/ img links will point to the dev images rather than the /cdn/deploy images, because they don't get rewritten, unlike the way we wrap img links in our PHP pages. We should hook the markdown parser and rewrite those to the corresponding /cdn/deploy image

(A longer-term alternative, look at building static html from our markdown files on deploy (or perhaps on first visit), so we can cache better)