enable lazy-loading of gitgraph.js (#9036)

- 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.

package.json

@@ -6,16 +6,20 @@
   },
   "devDependencies": {
     "@babel/core": "7.7.2",
+    "@babel/plugin-transform-runtime": "7.6.2",
     "@babel/preset-env": "7.7.1",
+    "@babel/runtime": "7.7.2",
     "autoprefixer": "9.7.1",
     "babel-loader": "8.0.6",
     "core-js": "3.4.1",
+    "css-loader": "3.2.0",
     "eslint": "6.6.0",
     "eslint-config-airbnb-base": "14.0.0",
     "eslint-plugin-import": "2.18.2",
     "less": "3.10.3",
     "less-plugin-clean-css": "1.5.1",
     "postcss-cli": "6.1.3",
+    "style-loader": "1.0.0",
     "stylelint": "11.1.1",
     "stylelint-config-standard": "19.0.0",
     "terser-webpack-plugin": "2.2.1",