Use babel minifier instead of uglify

[wbobeirne]

Some of our modules end up with ES6+ code in them as they're meant for a purely node environment. We could pull request and fix every problem, but it's killing our npm run build right now. As a quick stopgap for that, I've switched the prod webpack config to use babel-minify / babili now. In addition to handling those cases, I'm also seeing some huge file size improvement:

Old

Time: 32712ms

Asset                 Size          Chunks               Chunk Names
client.113ef751.js    4.88 MB       0  [emitted]  [big]  client
vendor.a1d0626c.js     196 kB       1  [emitted]         vendor

New

Time: 122573ms

Asset                 Size          Chunks               Chunk Names
client.9e40b3d7.js    3.42 MB       0  [emitted]  [big]  client
vendor.3a2c4d92.js     198 kB       1  [emitted]         vendor

Unfortunately, as you can see, it's noticeably slower. But we only build once per release, so I'd say it's worth it.