Twenty Thirteen: improve backward compatibility escape hatch for pre-3.6 versions:
 * Switch to the built-in method of providing a back link in `wp_die()` for the Customizer
 * Use WP_DEFAULT_THEME for both arguments during theme switching to simplify the logic
 * Prevent the 3.3 and earlier theme preview action gracefully

Props obenland for patches, fixes #24441.

git-svn-id: https://core.svn.wordpress.org/trunk@24411 1a063a9b-81f0-0310-95a4-ce76da25c4cd