Build process

[Sekhmet]

Fixes #1761
Fixes #1601

This should improve workflow when working on the server and make better test environment for E2E tests.

What has changed:

• development server uses SSR by default, so there shouldn't be problem with something working in development, but turns out to be broken with SSR.
• React components are hot reloadable (so state is preserved across changes).
• Reduce number of scripts to only three related to building (dev for development, build for building production app, and start to deploy it).
• SSR and client is hot reloadable - you don't have to wait for 2 minutes to test changes related to SSR.
• Migrated to Webpack 4.
• Migrated from ExtractTextPlugin to MiniCSSExtractPlugin.
• Reduced development bundle from 50MB to 2MB which makes remote debugging possible.

Changes

• Migrate to webpack 4.
• Add dev script with hot reloading on both server and client.
• Add build script.
• Remove old scripts and replace them with dev, build and start.
• Enable client-side HMR:
  • Make modules pure.
  • Enable HMR in webpack.
  • Add React HMR.
• Update Heroku build.
• Update Dockerfile.
• Update docs.

Test plan

dev script

Note: styles are loaded using style-loader on development, so there is blink of not styled content after refreshing. It's slight inconvenience, but it allows styles to be hot-reloadable.

• Checkout this branch.
• Run yarn.
• Run yarn dev.
• Wait for compilation to finish.
• Visit http://localhost:3000.
• Regular, server-side rendered page should show up.
• Make sure SSR works.
• Make some change to React component (for example change beta text inside Topnav.js)
• Text should update without full reload.
• Make some change to styles (for example change font-size for Topnav__brand)
• Style should update without full reload.
• Make some change to reducer (for example change one boolean value inside of feedReducer.js).
• Entire page should refresh.
• Make some change to server code (eg. comment app.get('/*', ssrHandler); out in app.js).
• Refresh the page.
• 404 page should show up (Cannot GET /).

build and start script

• Checkout this branch.
• Run yarn.
• Run yarn build.
• Wait for compilation to finish.
• Run yarn start.
• Visit http://localhost:3000.
• Regular, server-side rendered page should show up.
• Make sure SSR works.

Compare bundle size:
This PR: https://busy-master-pr-1825.herokuapp.com/statistics.html
Staging: https://staging.busy.org/js/statistics.html
Overall bundle size decreased by small amount (few KB).

[bonustrack]

Great work that simplify a lot stuff! Tested in local and everything seem to work well, SSR, hot reload, build etc

[jm90m]

works well for me now, even when making a change, their is no more memory leak