fix(src/site/webpack.config.js): don't generate unused index.css.js file

[nicholaschiang]

fix(src/site/webpack.config.js): don't generate unused index.css.js file

Right now, we have to specify an output JavaScript file for webpack to compile but we don't ever reference that file and it'd be nice to stop even generating it in the first place.

Instead, we use a file-loader along with an extract-loader to put all of our webpacked CSS into a separate file (so we concurrently load JavaScript and CSS instead of putting it all into index.js like we do with our app).