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.

templates/base/footer.tmpl

@@ -23,10 +23,6 @@
 		CodeMirror.modeURL =  "{{StaticUrlPrefix}}/vendor/plugins/codemirror/mode/%N/%N.js";
 	</script>
 {{end}}
-{{if .RequireGitGraph}}
-	<!-- graph -->
-	<script src="{{StaticUrlPrefix}}/vendor/plugins/gitgraph/gitgraph.js"></script>
-{{end}}
 
 <!-- Third-party libraries -->
 {{if .RequireHighlightJS}}