Resolve "Without JS+narrow screens, top menu bar does not work"

• This change makes the hamburger menu work without requiring JS and by only using CSS techniques
• It is implemented by using a technique referred to as checkbox hack: https://css-tricks.com/the-checkbox-hack/

Browser support seems to be very well: https://caniuse.com/#feat=mdn-css_selectors_checked

Tested it on the most recent version of Chrome, Firefox, Chrome on Android, Firefox on Android, Safari and it's even working on IE 😲

With this change we will be one of very few sites that have a working mobile menu when JS is disabled as far as I just learned when trying out a few popular sites. It's also an accessibility gain.

Closes #16 (closed)