Add `--no-js` option to validate.sh

I don't have the right versions of node on my laptop, and anyway don't want to spend the battery power running those tests most of the time.  This lets me run `common/validate.sh --no-js` for quick Python-only results.

Note that changes to the Python code *can* cause node failures, as the data API spec can change, so this isn't something we should be doing automatically.