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.

routers/repo/commit.go

@@ -111,7 +111,6 @@ func Graph(ctx *context.Context) {
 	ctx.Data["Reponame"] = ctx.Repo.Repository.Name
 	ctx.Data["CommitCount"] = commitsCount
 	ctx.Data["Branch"] = ctx.Repo.BranchName
-	ctx.Data["RequireGitGraph"] = true
 	ctx.Data["Page"] = context.NewPagination(int(allCommitsCount), setting.UI.GraphMaxCommitNum, page, 5)
 	ctx.HTML(200, tplGraph)
 }