Fix sidebar chunk spacing in two-column (desktop-mid) view

Set the top margin of #mw-related-navigation and its first sidebar chunk
to 0. This replicates the margin collapsing that occurs between other
sidebar chunks, so that all the gaps between sidebar chunks are the same
size.

Bug: T181256
Change-Id: I35b90383be8610412566e7367ecf25d771d4747c

resources/screen-desktop-mid.less

@@ -5,8 +5,8 @@
 	.column-left();
 }
 
-#mw-related-navigation {
-	margin-top: 2em;
+#mw-related-navigation > div.sidebar-chunk:first-child {
+	margin-top: 0;
 }
 
 #mw-content {