fix(rangeSlider): handles now support stacking (#1129)

It was already supported in noUiSlider, just we missed the right css
class.

Since noUiSlider is not really BEM, we cannot use the element
modifier() notation.

Author: vvo

src/css/default/_range-slider.scss

@@ -117,4 +117,7 @@ $range-slider-handle-bg: $white;
     }
   }
 
+  .ais-range-slider--stacking .ais-range-slider--handle {
+    z-index: 20;
+  }
 }