using codemirror json-ld mode from upstream

This removes the locally-hosted codemirror javascript mode and fetches the new, upstream-merged one in codemirror 3.22.0 from cdnjs. I've deleted the old version of the javascript mode, as it will no longer be maintained.

Also using the `min.css` version for all of the codemirror styles, since I actually took the time to make them.

If this is merged, I believe #321 can be closed.