Use Webpack 4 chunk prefetch to improve performance

Since we’ve upgraded to Webpack 4, we can take advantage of Webpack 4’s chunk prefetch/preload feature to improve our performance.

Documentation: https://webpack.js.org/guides/code-splitting/#prefetching-preloading-modules
Search for \bimport\( to find places where we use code splitting.

This is not very high on our priority list because we already use service worker to do prefetch, but it’s a simple thing to do.

[ZhangYiJiang]

As the docs say

This will result in <link rel="prefetch" href="login-modal-chunk.js"> being appended in the head of the page

So you can run yarn build and see the results. You can also open a PR on this repo - we have a bot that automatically run and deploy preview builds of PR branches.

[masihtamsoy]

I have included webpackPreload to import(), as suggested on files. I am not confident on how should I test this and deliver. Should I test it using webpack-bundle-analyzer? How should I proceed?