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/vendor/gitgraph.js/gitgraph.custom.css

@@ -0,0 +1,13 @@
+#git-graph-container, #rel-container {float:left;}
+#rel-container {max-width:30%; overflow-x:auto;}
+#git-graph-container {overflow-x:auto; width:100%}
+#git-graph-container li {list-style-type:none;height:20px;line-height:20px; white-space:nowrap;}
+#git-graph-container li .node-relation {font-family:'Bitstream Vera Sans Mono', 'Courier', monospace;}
+#git-graph-container li .author {color:#666666;}
+#git-graph-container li .time {color:#999999;font-size:80%}
+#git-graph-container li a {color:#000000;}
+#git-graph-container li a:hover {text-decoration:underline;}
+#git-graph-container li a em {color:#BB0000;border-bottom:1px dotted #BBBBBB;text-decoration:none;font-style:normal;}
+#rev-container {width:100%}
+#rev-list {margin:0;padding:0 5px 0 5px;min-width:95%}
+#graph-raw-list {margin:0px;}