Turn multi page app into single page app

Soooo, this is a rather huge PR. It does several things:

1. Moves all third-party JS from static/js to static/js/lib
1. Moves all non-third-party JavaScript from static/js to client/js
1. Splits the JS into several files (one for each controller etc.)
1. Introduce Gulp as build system to lint the files via ESLint, and more important, concatenate and minify all non-third-party JS code to one file (static/js/app.js and static/js/app.min.js respectively). I've added app.js/app.min.js to the repo for now but they should probably be removed later and created only locally via gulp.
1. Introduce angular-routes. All former pages (index, dashboard, list/add employees, list/add customers, help) are now views. There is only one index.html left which has an ng-view div. So this turns this into a proper single page app.

Especially the last change is a somewhat fundamental change. I wasn't sure if you intended to keep each page as its own, server-side rendered page - but if so, using angular makes little sense?

We can also have a look at this together and discuss it face to face before you merge this.

One more thing: There's a `package.json` in the `client` folder now and you have to run `npm install` there to install gulp and the gulp plugins. Also `npm i -g gulp` is advisable (to have the gulp binary in the path). Finally, to work on the frontend code you should let `gulp watch` run all the time. For building the frontend once, `gulp` suffices.