Split code into chunks to decrease main bundle size

[arildm]

The main bundle is now 553 KiB when gzipped, while Webpack recommends no more than 244 KiB.

$ yarn build
...
WARNING in asset size limit: The following asset(s) exceed the recommended size limit (244 KiB).
This can impact web performance.
Assets: 
  bundle.js (1.98 MiB)
  b1a653db79258eeccc5d.ttf (388 KiB)
  bundle.js.gz (553 KiB)
...

With code splitting we should be able to decrease that and thus make first loads faster.

Maybe related:

• Add content hash to bundle.js to fix caching #318