Fix oversized loading overlay on gas customization modal. (#6326)

Author: danjm

ui/app/components/app/gas-customization/gas-modal-page-container/advanced-tab-content/index.scss

@@ -63,6 +63,10 @@
       font-size: 10px;
       color: #888EA3;
     }
+
+    .loading-overlay {
+      height: auto;
+    }
   }
 
   &__slider-container {