Theme Customizer: Fire WP_Customize->customize_preview_init on wp_loaded...

Theme Customizer: Fire WP_Customize->customize_preview_init on wp_loaded instead of template_redirect. props nacin, see #19910.

Certain customize settings (such as show_on_front and other static front page features) need to be overridden before template_redirect.

git-svn-id: https://core.svn.wordpress.org/trunk@20306 1a063a9b-81f0-0310-95a4-ce76da25c4cd