Upgrade (or replace) codemirror uses

CodeMirror 6 has launched, which is a pretty substantial upgrade from CodeMirror 5. That being said, it's not a drop-in replacement, or an easy migration path. We should take the time to visit all our uses of codemirror and upgrade to CodeMirror 6. ...or upgrade to something else entirely. In the meantime, [the Monaco editor]( has become way more stable and feature rich as well, which could be a good alternative.