Slim the webpack build

[saiichihashimoto]

Currently, there's four instances of webpack running. Ideally, there'd be one.

Initially, I wanted for background.js (or top-frame.js, etc.) to be able to require('./module') and it would include require('./module.background') if it existed. With the multiple entry chunks, this would mean multiple sets of resolve.extensions, which webpack/webpack#2801 isn't down with. I solved this with running webpack multiple time. This is horrible for performance.