Fix Media field lightbox in subform (#11760)

This PR fixes a media field lightbox issue when additional subforms are added extending the page height. This results in the lightbox background no longer filling the page. Setting the lightbox background div (#sbox-overlay) to 'fixed' ensures the background remains within the viewport regardless of the page height.

media/system/css/modal.css

@@ -13,7 +13,7 @@
  */
 
 #sbox-overlay {
-	position: absolute;
+	position: fixed;
 	background-color: #000;
 	left: 0px;
 	top: 0px;