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/gitGraph.js

@@ -0,0 +1,16 @@
+$(async () => {
+  const graphCanvas = document.getElementById('graph-canvas');
+  if (!graphCanvas) return;
+
+  const [{ default: gitGraph }] = await Promise.all([
+    import(/* webpackChunkName: "gitgraph" */'../vendor/gitgraph.js/gitgraph.custom.js'),
+    import(/* webpackChunkName: "gitgraph" */'../vendor/gitgraph.js/gitgraph.custom.css'),
+  ]);
+
+  const graphList = [];
+  $('#graph-raw-list li span.node-relation').each(function () {
+    graphList.push($(this).text());
+  });
+
+  gitGraph(graphCanvas, graphList);
+});