Configure npm build to reference absolute paths

Running `npm deploy-docs` is otherwise broken and requires manually rewriting it 🙃 https://github.com/mapbox/mr-ui/commit/2c6250712bd4ea4f17d453c3c1988a32254ddf7e

I'm sure there's a configuration step in https://www.npmjs.com/package/react-app-rewired to do that.