Make navigation scrollable when browser too narrow

This adds scrollbars to the navigation when the browser is too narrow but too
wide for the mobile version of the menu. This prevents the navigation from
wrapping around and messing up the design.

assets/css/index.css

@@ -59,6 +59,8 @@ body {
     margin: 0 auto;
     bottom: 0;
     width: 100%;
+    white-space: nowrap;
+    overflow-x: auto;
 }
 
 .header-subnav li {