enable lazy-loading of gitgraph.js (go-gitea#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.

.eslintignore

@@ -1 +1,2 @@
 /public/js/semantic.dropdown.custom.js
+/web_src/js/vendor/**

.eslintrc

@@ -14,6 +14,7 @@ env:
  node: true
 
 globals:
+ __webpack_public_path__: true
  Clipboard: false
  CodeMirror: false
  Dropzone: false