Fix CodeMirror editor options

## Bug Description

The editor options are only working because we're recreating CodeMirror. If we fix it not to do that (as that has other unfortunate consequences) then the options stop working.

This is what we're trying to do at a low-level with CodeMirror:

https://codesandbox.io/s/dank-currying-2rjxr

but it's not taking effect (either in that sandbox or our editor).