Bug 1792834: Adjust the btn-group-top z-index

So that btn-group-top z-index is higher than outer-link z-index

behatnotneeded

Change-Id: Ide6fde8a22035266a5706929163c01028d250bf5
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>

htdocs/theme/raw/sass/components/_btn.scss

@@ -131,7 +131,7 @@ h1 ~ div > .btn-group-top,
 
 .btn-group-top {
     position: relative;
-    z-index: 1;
+    z-index: 3;
     @media (min-width: $screen-xs-min) {
         float: right;
         .btn {