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/pwa/serviceworker_js.tmpl

@@ -6,9 +6,9 @@ var urlsToCache = [
   '{{StaticUrlPrefix}}/vendor/plugins/jquery-migrate/jquery-migrate.min.js?v=3.0.1',
   '{{StaticUrlPrefix}}/vendor/plugins/semantic/semantic.min.js',
   '{{StaticUrlPrefix}}/js/index.js?v={{MD5 AppVer}}',
+  '{{StaticUrlPrefix}}/js/gitgraph.js?v={{MD5 AppVer}}',
   '{{StaticUrlPrefix}}/js/semantic.dropdown.custom.js?v={{MD5 AppVer}}',
   '{{StaticUrlPrefix}}/vendor/plugins/clipboard/clipboard.min.js',
-  '{{StaticUrlPrefix}}/vendor/plugins/gitgraph/gitgraph.js',
   '{{StaticUrlPrefix}}/vendor/plugins/vue/vue.min.js',
   '{{StaticUrlPrefix}}/vendor/plugins/emojify/emojify.custom.js',
   '{{StaticUrlPrefix}}/vendor/plugins/cssrelpreload/loadCSS.min.js',
@@ -25,7 +25,6 @@ var urlsToCache = [
   '{{StaticUrlPrefix}}/vendor/assets/font-awesome/css/font-awesome.min.css',
   '{{StaticUrlPrefix}}/vendor/assets/octicons/octicons.min.css',
   '{{StaticUrlPrefix}}/vendor/plugins/simplemde/simplemde.min.css',
-  '{{StaticUrlPrefix}}/vendor/plugins/gitgraph/gitgraph.css',
   '{{StaticUrlPrefix}}/vendor/plugins/tribute/tribute.css',
   '{{StaticUrlPrefix}}/vendor/plugins/semantic/semantic.min.css',
   '{{StaticUrlPrefix}}/css/index.css?v={{MD5 AppVer}}',