Coding Standards: Move wp-admin/custom-header.php to wp-admin/includes/class-custom-image-header.php

This renames the file containing the `Custom_Image_Header` class to conform to the coding standards.

This commit also includes:
- A new `custom-header.php` that includes the new file, for anyone that may've been including the file directly.
- Replaces references to the old filename with the new filename.

See #47632.


Built from https://develop.svn.wordpress.org/trunk@45654


git-svn-id: http://core.svn.wordpress.org/trunk@45465 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/theme.php

@@ -2604,7 +2604,7 @@ function _custom_header_background_just_in_time() {
 		}
 
 		if ( is_admin() ) {
-			require_once( ABSPATH . 'wp-admin/custom-header.php' );
+			require_once( ABSPATH . 'wp-admin/includes/class-custom-image-header.php' );
 			$custom_image_header = new Custom_Image_Header( $args[0]['admin-head-callback'], $args[0]['admin-preview-callback'] );
 		}
 	}