Add fallback featured image (#2555)

* 💄 Reorder filters and actions alphabetically

* Add a default src for featured images and og:image tags

* Use versioning instead of a new image

Follows the existing practice

* Remove jetpack default og:image filter

This isn't called if the site has a site icon configured, which prod has

wp-content/themes/pub/wporg-learn-2024/functions.php

@@ -18,18 +18,21 @@
  */
 add_action( 'after_setup_theme', __NAMESPACE__ . '\setup' );
 add_action( 'wp_enqueue_scripts', __NAMESPACE__ . '\enqueue_assets' );
-add_filter( 'wporg_block_site_breadcrumbs', __NAMESPACE__ . '\set_site_breadcrumbs' );
-add_filter( 'wporg_block_navigation_menus', __NAMESPACE__ . '\add_site_navigation_menus' );
-add_filter( 'single_template_hierarchy', __NAMESPACE__ . '\modify_single_template' );
-remove_filter( 'template_include', array( 'Sensei_Templates', 'template_loader' ), 10, 1 );
-add_filter( 'sensei_register_post_type_lesson', function( $args ) {
-	$args['has_archive'] = 'lessons';
-	return $args;
-} );
+
+add_filter( 'post_thumbnail_html', __NAMESPACE__ . '\set_default_featured_image', 10, 5 );
 add_filter( 'sensei_register_post_type_course', function( $args ) {
 	$args['has_archive'] = 'courses';
 	return $args;
 } );
+add_filter( 'sensei_register_post_type_lesson', function( $args ) {
+	$args['has_archive'] = 'lessons';
+	return $args;
+} );
+add_filter( 'single_template_hierarchy', __NAMESPACE__ . '\modify_single_template' );
+add_filter( 'wporg_block_navigation_menus', __NAMESPACE__ . '\add_site_navigation_menus' );
+add_filter( 'wporg_block_site_breadcrumbs', __NAMESPACE__ . '\set_site_breadcrumbs' );
+
+remove_filter( 'template_include', array( 'Sensei_Templates', 'template_loader' ), 10, 1 );
 
 /**
  * Modify the single template hierarchy to use customised copies of the Sensei Course Theme templates.
@@ -279,3 +282,21 @@ function set_site_breadcrumbs( $breadcrumbs ) {
 
 	return $breadcrumbs;
 }
+
+/**
+ * Set the default featured image.
+ *
+ * @param string       $html The HTML for the featured image.
+ * @param int          $post_id The post ID.
+ * @param int          $post_thumbnail_id The post thumbnail ID.
+ * @param string|array $size The image size.
+ * @param string|array $attr The image attributes.
+ * @return string The modified HTML for the featured image.
+ */
+function set_default_featured_image( $html, $post_id, $post_thumbnail_id, $size, $attr ) {
+	if ( ! $html ) {
+		return '<img src="https://s.w.org/images/learn-thumbnail-fallback.jpg?v=4" alt="" />';
+	}
+
+	return $html;
+}