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.

public/vendor/librejs.html

@@ -46,7 +46,7 @@
           <td><a href="https://github.com/zenorocha/clipboard.js/archive/v1.5.9.tar.gz">clipboard-1.5.9.tar.gz</a></td>
         </tr>
         <tr>
-          <td><a href="./plugins/gitgraph/gitgraph.js">gitgraph.js</a></td>
+          <td><a href="../js/gitgraph.js">gitgraph.js</a></td>
           <td><a href="https://github.com/bluef/gitgraph.js/blob/master/LICENSE">BSD 3-Clause</a></td>
           <td><a href="https://github.com/bluef/gitgraph.js">gitgraph.js-latest</a></td>
         </tr>