UI: Dashboard tweaks (#5974)

- Don't show divider below heatmap when it's hidden via media-query
- Make divider between user menu adhere to container width
- Remove excessiv margins on heatmap

public/less/_base.less

@@ -598,10 +598,9 @@ footer {
     margin-bottom: 2px !important;
 }
 
-#user-heatmap{
+#user-heatmap {
     width: 107%; // Fixes newest contributions not showing
     text-align: center;
-    margin: 40px 0 30px;
 
     svg:not(:root) {
         overflow: inherit;