Fix line-height in admin menu items

When items went over multiple lines, the distance between their lines
was the same as the distance between one element and another one. This
made it hard to differentiate how many elements there were.

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

app/assets/stylesheets/admin/menu.scss

@@ -142,8 +142,7 @@
   li a {
     color: inherit;
     display: block;
-    line-height: $line-height * 2;
+    padding: $line-height / 2 $line-height / 4;
-    padding-left: $line-height / 4;
     vertical-align: top;
 
     &:hover {