Extend the list header top border to the edge of the box

With border board lists (those created from labels) the top border
created bad visual artifacts with the .board-inner gray border.
With this change the colored border overflows the padding-box of
.board-inner and extends to the edges.
It works very well with most colors, excepts for very light ones (such
as `#ff0` yellow for instance).

app/assets/stylesheets/pages/boards.scss

@@ -109,6 +109,12 @@
 
   &.has-border {
     border-top: 3px solid;
+    margin-top: -1px;
+    margin-right: -1px;
+    margin-left: -1px;
+    padding-top: 1px;
+    padding-right: 1px;
+    padding-left: 1px;
 
     .board-title {
       padding-top: ($gl-padding - 3px);