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)

_includes/header.html

@@ -46,7 +46,7 @@ See also:
             The associated CSS will then display the navigation elements as a
             menu.
         {% endcomment %}
-        <input type="checkbox" id="mobile_toggle" style="display:none;">
+        <input type="checkbox" id="mobile_toggle" class="d-none">
         <label for="mobile_toggle" class="nav_toggle"></label>
 
         <nav>