Add button when scrolled outside of visible area (#643)

With the infinite scroll behavior, it's easy to scroll super far away from where the content is and have a hard time getting back. This PR adds a button to refocus on the center of the scene when no elements are visible anymore.

src/styles.scss

@@ -256,3 +256,11 @@ button,
   clip: rect(1px, 1px, 1px, 1px);
   white-space: nowrap; /* added line */
 }
+
+.scroll-back-to-content {
+  position: fixed;
+  left: 50%;
+  bottom: 20px;
+  transform: translateX(-50%);
+  padding: 10px 20px;
+}