Unify Foundation classes in header menus

In commit 55c3c7987 we updated the admin layout header to use
`class="dropdown menu" data-dropdown-menu` instead of `class="menu"
data-responsive-menu="medium-dropdown"`.

Then, in commit dcec003d0, we updated the public layout header to use
`class="menu" data-responsive-menu="medium-dropdown"` instead of
`class="dropdown menu" data-dropdown-menu`.

Now we're applying the same classes to both, hoping we can get some
consistent styles.

I'm choosing to keep the ones in the public layout because using it I
had less problems when trying to improve the styles of this navigation.

app/views/layouts/_admin_header.html.erb

@@ -40,7 +40,7 @@
 
       <div id="responsive_menu">
         <div class="top-bar-right">
-          <ul class="dropdown menu" data-dropdown-menu>
+          <ul class="menu" data-responsive-menu="medium-dropdown">
             <%= render "shared/admin_login_items" %>
             <%= render "layouts/notification_item" %>
             <%= render "devise/menu/login_items" %>