Make header responsive (fix #11)

I'm proposing a new responsive header. It works fine on desktop and mobile, and uses flexbox and media queries to switch the menu direction on smaller screens (from horizontal to vertical).

I introduced _new.scss. There lie the new styles (for responsive parts) replacing some parts of _leap.scss. I'm proposing to incrementally add responsiveness-related changes to this file to keep track of them, and only once the website is fully responsive and we're satisfied, remove the cruft and deprecate old styles.

Feel free to take/edit what you please, and don't hesitate to give me feedback. If you notice anything broken on an old/weird browser, this is fully unintended so please report it and we can do our best to fix that.