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.

web_src/js/publicPath.js

@@ -0,0 +1,12 @@
+/* This sets up webpack's chunk loading to load resources from the same
+  directory where it loaded index.js from. This file must be imported
+  before any lazy-loading is being attempted. */
+
+if (document.currentScript && document.currentScript.src) {
+  const url = new URL(document.currentScript.src);
+  __webpack_public_path__ = `${url.pathname.replace(/\/[^/]*$/, '')}/`;
+} else {
+  // compat: IE11
+  const script = document.querySelector('script[src*="/index.js"]');
+  __webpack_public_path__ = `${script.getAttribute('src').replace(/\/[^/]*$/, '')}/`;
+}