Fix all submenus expanded recursively in dropdown

The problem was that the CSS selector made *all* of a submenu's nested
menus visible. Fixed by applying the selector to the immediate nested
menu only.

less/dropdowns.less

@@ -172,7 +172,7 @@
      -moz-border-radius: 0 6px 6px 6px;
           border-radius: 0 6px 6px 6px;
 }
-.dropdown-submenu:hover .dropdown-menu {
+.dropdown-submenu:hover > .dropdown-menu {
   display: block;
 }