webpack polyfills did not work because useBuiltIns: 'entry' expects a
explicit core-js import. Changed it to 'usage' which does not require
these explicit imports and polyfills based on browserslist.

As a result, the built index.js now went from 128kB to 192kB.

Ref: https://babeljs.io/docs/en/babel-preset-env#usebuiltins

Co-authored-by: zeripath <art27@cantab.net>

- moved gitgraph.js to web_src and made it importable and es6-compatible
- created new webpack chunk for gitgraph
- enabled CSS loader in webpack
- enabled async/await syntax via regenerator-runtime
- added script to ensure webpack chunks are loaded correctly
- disable terser's comment extraction to prevent .LICENCE files

gitgraph.js has many issues:

1. it is incompatible with ES6 because of strict-mode violations
1. it does not export anything
1. it's css has weird styles like for `body`
1. it is not available on npm

I fixed points 1-3 in our version so it's now loadable in webpack. We should eventually consider alternatives.

* pack web_src/js/draw.js to public/js/index.js

* fix eslint

* modernize js and use babel

- add babel toolchain to transform modern JS to ES5
- extend eslint config for modern rules
- fixes linting issues via `eslint --fix` and manual fixes

* run 'make css' to satisfy CI

* code style tweaks and set js indendation to 2 in .editorconfig

* regenerate js

* Move index.js to web_src and use webpack

* Fix initHeatMap

* update eslint and move webpack to devDependencies

* update index.js

* add eslint env node