Increase menu button touch area on small screens

Some users might not be able to touch the icon due to a motor
disability. Other users might think the "Menu" text is part of the
button and try to touch it instead.

Making the "Menu" text part of the button makes it easier to show/hide
this menu. Besides, it lets screen reader users with a small screen hear
the word "Menu" associated to the button.

We could simplify the HTML a bit more but Foundation's `hamburger` mixin
uses the `::after` element with `position: absolute`, so we can't apply
it directly to the button without making the CSS more complex.

app/assets/stylesheets/layout.scss

@@ -667,9 +667,12 @@ body > header,
   }
 }
 
+.menu-button {
+  color: inherit;
+}
+
 .menu-icon {
   @include hamburger($color: currentcolor, $color-hover: currentcolor);
-  color: inherit;
 }
 
 .title-bar {