Get code to build with webpack

[JacobEvelyn]

It looks like @kavisherlock and I are debugging the same issue as described in #108. I updated a bunch of things in package.json to get npm run dev to work, which all seem like relatively safe changes to me.

I'm less confident that the changes I made to build/utils.js and build/webpack.prod.conf.js are safe—that was more me trying to follow Webpack 3 -> 4 guides and deleting things I didn't understand. But at least npm run build now works for me!

Fixes #108