Fix + rework KHG mensa parser, #133

[rnbwdsh]

Refactor KHGMenuLoader to

• work again (new url)
• use the url just once from Consts
• Move column-handler to separate method so the the line isn't indent-lvl5
• single css query selector instead of mix between query and getter-calling

[marunjar]

Perfect, thanks. But can you remove c06212c from this PR, this is not really related ;)