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/_base.scss

@@ -290,6 +290,8 @@ nav {
   }
 
   a {
+    display: block;
+    padding-bottom: 0.25em;
     text-decoration: none;
   }