Add a border to the menu button

This way we make it more obvious that it's clickable and, since adding a
border requires adding padding, we increase the touch area, making it
easier to use and more accessible for users with motor disabilities.

Since now the button looks like a button, we don't need the pointer
cursor Foundation adds to hamburger icons and can use the default cursor
browsers provide for buttons.

app/assets/stylesheets/layout.scss

@@ -668,11 +668,15 @@ body > header,
 }
 
 .menu-button {
+  border: 1px solid;
+  border-radius: $button-radius;
   color: inherit;
+  padding: 0.6em;
 }
 
 .menu-icon {
   @include hamburger($color: currentcolor, $color-hover: currentcolor);
+  cursor: inherit;
 }
 
 .title-bar {