Site Icon: Add Customizer UI.

Second part of the Site Icon feature after [32994] introduced it for Settings.

Props celloexpressions.
See #16434.



git-svn-id: https://develop.svn.wordpress.org/trunk@33154 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/general-template.php

@@ -2445,7 +2445,7 @@ function wp_no_robots() {
  * @link http://www.whatwg.org/specs/web-apps/current-work/multipage/links.html#rel-icon HTML5 specification link icon.
  */
 function wp_site_icon() {
-	if ( ! has_site_icon() ) {
+	if ( ! has_site_icon() && ! is_customize_preview() ) {
 		return;
 	}