Update CommonMarker

[pushcx]

We’re on a years-old version of CommonMarker. It’s changed its API, dropping a method we used in extras/markdowner.rb to add attributes to links, rewrite some HTML, and make the @username feature work. (Heck, they even renamed their constant from CommonMarker to Commonmarker.)

That issue links to a gem they suggest, but perhaps we should just expand our use of Nokogiri? Or maybe there’s a very different way to tie into the renderer at the time it’s emitting HTML, which I’d be more confident in than re-parsing and fixing things up externally. So this one requires some technical investigation!

[pushcx]

I'm not sure the check that a user exists is worthwhile given that users can change their usernames pretty freely.

Maybe our username linking can become an extension to have a more stable API?