WordPress gallery: enable linking images to different URLs

gistfile1.php

<?php

class Gallery_Images_Links {

	public function __construct() {
		add_filter( 'attachment_link', array( $this, 'attachment_link' ), 10, 2 );
		add_filter( 'attachment_fields_to_edit', array( $this, 'attachment_fields_to_edit' ), 20, 2 );
		add_filter( 'attachment_fields_to_save', array( $this, 'attachment_fields_to_save' ), 10, 2 );
	}

	function attachment_link( $link, $id ) {
		if( $custom_link = get_post_meta( $id, '_link', true ) ) {
			$link = $custom_link;
		}

		return $link;
	}

	function attachment_fields_to_edit( $fields, $post ) {
		$fields['link'] = array(
			'label' => __( 'Link' ),
			'input' => 'text',
			'value' => get_post_meta( $post->ID, '_link', true ),
		);

		return $fields;
	}

	function attachment_fields_to_save( $post, $attachment ) {
		if( isset( $attachment['link'] ) ) {
			update_post_meta( $post['ID'], '_link', $attachment['link'] );
		}

		return $post;
	}
}
new Gallery_Images_Links;