Replace all instances of thickbox theme preview with the theme customizer. fixes #20404.

* Use theme customizer in theme install/update screens.
* Separate the customize loader from the customizer. Use wp_customize_loader() to include the loader script and markup.
* Deprecated: wp-admin/js/theme-preview.js is now no longer used by core.

git-svn-id: https://core.svn.wordpress.org/trunk@20419 1a063a9b-81f0-0310-95a4-ce76da25c4cd