Fix margin for attached top header on code review (#11571)

Introduced naively by #11463

The margin was being applied too widely.

web_src/less/_review.less

 
 .add-comment-left.add-comment-right .ui.attached.header {
     border: 1px solid #d4d4d5;
-    margin-bottom: .5em;
     margin-top: .5em;
+
+    &:not(.top) {
+        margin-bottom: .5em;
+    }
 }
 
 .focus-lines-new .ui.button.add-code-comment.add-code-comment-right,