Add a placeholder skeleton

src/cryptometrics/components/skeletons/PlaceholderSkeleton.js

+import React from "react";
+import classNames from "classnames";
+
+function PlaceholderSkeleton({ className }) {
+  return (
+    <span
+      className={classNames(
+        `skeleton-box inline-block rounded-md ${className}`
+      )}
+    ></span>
+  );
+}
+
+export default PlaceholderSkeleton;