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/index.js

@@ -2,6 +2,9 @@
 /* exported timeAddManual, toggleStopwatch, cancelStopwatch, initHeatmap */
 /* exported toggleDeadlineForm, setDeadline, updateDeadline, deleteDependencyModal, cancelCodeComment, onOAuthLoginClick */
 
+import './publicPath';
+import './gitGraph';
+
 function htmlEncode(text) {
   return jQuery('<div />').text(text).html();
 }