Use DejaVu Sans fonts

Courier which is used on this website (if you have Wine installed, which lets you run Windows programs) is the ugly font (and I hate it). This pull request uses DejaVu Sans Mono if possible, instead of Courier, which is in my opinion way better font.

This font exists in most Linux distributions.

![DejaVu Sans Mono](https://upload.wikimedia.org/wikipedia/commons/b/b9/DejaVu_Sans_Mono.svg)