Move z-index ordering

Author: KoolADE85

components/dash-core-components/src/components/css/sliders.css

@@ -50,6 +50,8 @@
 }
 
 .dash-slider-thumb {
+    position: relative;
+    z-index: 1;
     display: block;
     width: 16px;
     height: 16px;
@@ -172,7 +174,6 @@
 .dash-slider-wrapper {
     flex: 1;
     min-width: 0;
-    z-index: 1;
 }
 
 .dash-range-slider-inputs {