Skatepark: Add space below the Featured Image on the single post page (#5061)

The logic behind the calc() code is trying to mimic the way the space gets calculated when a Featured Image isn't set

skatepark/block-templates/single.html

@@ -9,7 +9,7 @@
 
 <!-- wp:group {"align":"full"} -->
 <div class="wp-block-group alignfull">
-<!-- wp:post-featured-image {"align":"full"} /--></div>
+<!-- wp:post-featured-image {"align":"full", "style":{"spacing":{"margin":{"bottom":"calc(2*var(--wp--style--block-gap) + 3*var(--wp--custom--gap--horizontal))"}}}} /--></div>
 <!-- /wp:group -->
 
 <!-- wp:post-content {"layout":{"inherit":true}} /-->