Update social links block to output a custom class if it is present o…

…n the individual social link items

Add comment explaining the MutationObserver stub in the storybook test

packages/block-library/src/social-link/index.php

@@ -19,14 +19,15 @@ function render_block_core_social_link( $attributes ) {
 		$attributes['label'] :
 		/* translators: %s: Social Link service name */
 		sprintf( __( 'Link to %s' ), block_core_social_link_get_name( $service ) );
+	$class_name = ( isset( $attributes['className'] ) ) ? ' ' . $attributes['className'] : false;
 
 	// Don't render a link if there is no URL set.
 	if ( ! $url ) {
 		return '';
 	}
 
 	$icon = block_core_social_link_get_icon( $service );
-	return '<li class="wp-social-link wp-social-link-' . esc_attr( $service ) . '"><a href="' . esc_url( $url ) . '" aria-label="' . esc_attr( $label ) . '"> ' . $icon . '</a></li>';
+	return '<li class="wp-social-link wp-social-link-' . esc_attr( $service ) . esc_attr( $class_name ) . '"><a href="' . esc_url( $url ) . '" aria-label="' . esc_attr( $label ) . '"> ' . $icon . '</a></li>';
 }
 
 /**