Menu Customizer: first-pass at using a JS template for menu items. Currently this only works for existing items - dynamically-rendered items need to be adjusted to use this, and then `menu_customizer_render_item_control()` can be entirely removed. Minimal testing so far, but it looks like some things with previewing will need to be adjusted to account for the new way this works. New menu items were already broken before this commit but will be reworked anyway to leverage this.

This makes menu items scale really well, because all html structure for menu item UI is created in JS from a single template rather than being sent down from the server.

See #2278.

git-svn-id: https://plugins.svn.wordpress.org/menu-customizer/trunk@1039712 b8457f37-d9ea-0310-8a92-e5e31aec5664