Add new worker for zipping outputs.

Use bakeId to track which outputs are stale.

src/web/stylesheets/layout/_controls.css

@@ -49,3 +49,15 @@
 #controls .btn {
     border-radius: 30px;
 }
+
+.spin {
+    animation-name: spin;
+    animation-duration: 3s;
+    animation-iteration-count: infinite;
+    animation-timing-function: linear;
+}
+
+@keyframes spin {
+    0%      {transform: rotate(0deg);}
+    100%    {transform: rotate(360deg);}
+}