Fix "Appearance -> Customize" always showing up as "new" and losing menu item settings.

In WP 4.0+, the URL of the "Customize" menu is different on every page because it includes the current page URL. As a result, AME doesn't realize it's the same menu and can't apply custom user settings. Fixed by removing the query parameter "return" from the template ID.

git-svn-id: https://plugins.svn.wordpress.org/admin-menu-editor/trunk@987089 b8457f37-d9ea-0310-8a92-e5e31aec5664