Lazy load images (#4228)

* Add lazy loading for many images
* Load sprites on first hover of scrubber

ui/v2.5/src/components/Shared/GridCard.tsx

@@ -85,6 +85,7 @@ export const GridCard: React.FC<ICardProps> = (props: ICardProps) => {
     if (props.interactiveHeatmap) {
       return (
         <img
+          loading="lazy"
           src={props.interactiveHeatmap}
           alt="interactive heatmap"
           className="interactive-heatmap"