feat: press enter or arrows to navigate

Not so much code, but the library `mgo-mousetrap` has lot of lines!

We can now navigate in the quizz with left arrow to go to previous screen (if allowed) and right arrow or enter to go to the next screen (next question or results). As the directive does not seem to work with conditions, I had to encapsulate the behaviour in two new methods.

Give it a shot and tell me what you think.