make appearance of lightbox configurable

static/js/snap-gallery.js

@@ -9,7 +9,11 @@ function openLightbox(id) {
   document.getElementById(lightbox_baseid + id).style.display = "block";
 
   // Kill automatic slideshow when lightbox opened
-  clearInterval(autoSlideshow);
+  try {
+    clearInterval(autoSlideshow);
+  } catch (e) {
+    console.log("Lightbox error: " + e)
+  }
 }
 
 // Close the Lightbox