Correct some lint on server side

Pylint complains about some little formatting that can indeed make the
code a little better. Other remarks were not corrected because they are
cosmetic and it's nicer like it is now.
Rewrite the client side controller in CoffeeScript

The client side controller has been rewritten using CoffeeScript. The
`app.js` is still versioned so the application is ready to be deployed
without the need of a CoffeeScript compiler.

The `app.js` can be regenerated with `coffee -c -o static/js
src/coffee/app.coffee`.