Precompress and optimize static files during build and serve them to browsers

[gmokki]

The PR is done - it:

• follows the style and naming rules (passes npm run lint)
• doesn't break anything (passes npm run test-local and npm run test-browserstack)
• any changed files are transformed to ES6

Changes:

• create precompressed variants of js, css, svg files: gz using zopfli and br using brotli

Currently using forked versions of expressjs/send/negotiator while waiting for the PRs to go through:

• Change resolved Accept-Encoding to honour server side order preferece jshttp/negotiator#49
• Add support for precompressed (for example gzip) content pillarjs/send#108

[hannesj]

Is there some reason not to use a ready-made plugin like https://webpack.js.org/plugins/compression-webpack-plugin/ for the compression of the assets?

[gmokki]

Only familiarity of basic unix tools.

I can try to use webpack plugins:

• https://webpack.js.org/plugins/compression-webpack-plugin/ for zopfli compression
• https://github.com/ngsru/brotli-webpack-plugin for the brotli compression
• https://www.npmjs.com/package/imagemin-webpack-plugin for pngquant, svgo
• and somehow figure out how to add https://github.com/imagemin/imagemin-zopfli to the imagemin-webpack-plugin

Or maybe I will just leave the image optimizations out of this PR...

[hannesj]

I think the imagemin-zopfli plugin can be added using this

[hannesj]

Also check #1031 where we have included the svgs to be bundled using webpack.

[hannesj]

Implemented the serving using a different express middleware, so we can use the vanilla express version.

See cc2c7f0