Increase amount of vertical whitespace below "Feedback" section.

This was mainly done to improve the appearance of DnDv2 exercises in
Apros but the appearance of DnDv2 exercises in the LMS benefits from it
as well.

drag_and_drop_v2/public/css/drag_and_drop.css

@@ -267,6 +267,7 @@
 /*** FEEDBACK ***/
 
 .xblock--drag-and-drop .feedback {
+    margin-bottom: 1%;
     border-top: solid 1px #bdbdbd;
 }