[bugfix] missing spinner in explore view (#914)

Author: mistercrunch

caravel/assets/stylesheets/caravel.css

@@ -149,6 +149,9 @@ div.navbar {
 }
 img.loading {
   width: 40px;
+  position: relative;
+  z-index: 10;
+  margin: 10px;
 }
 img.viz-thumb-option {
     width: 100px;