Fix line height in subnavigation items

When, on small screens, a navigation element had a very long text
causing it to span over multiple lines, the space between each line was
the same as the space between elements. This made it hard to see where
elements started and ended.

Using a padding to separate the contents of one element and the contents
of the next one solves the issue.

app/assets/stylesheets/layout.scss

@@ -744,7 +744,8 @@ body > header,
   a {
     color: inherit;
     display: inline-block;
-    line-height: $line-height * 2;
+    padding-bottom: $line-height / 2;
+    padding-top: $line-height / 2;
     position: relative;
     text-align: left;
     width: 100%;