Collapse sidebar navigation list on smaller screens

- CSS only solution using "check-box hack"
- Add new UI text label `menu_label` in _data/ui-text.yml
- Close #607

_sass/_sidebar.scss

@@ -33,10 +33,6 @@
     font-family: $sans-serif-narrow;
   }
 
-  h3, h4 {
-    font-size: $type-size-5;
-  }
-
   p, li {
     font-family: $sans-serif;
     font-size: $type-size-6;