TTK-21402 s/loadeddata/canplaythrough/g

http://automatist.org/blog/wp-content/uploads/2013/07/readyState2.png
https://www.w3.org/2010/05/video/mediaevents.html

src/Pumukit/NewAdminBundle/Resources/views/Pic/generate.html.twig

@@ -174,7 +174,7 @@
         if (video.readyState == 4) {
             init_extractpic();
         } else {
-            video.addEventListener("loadeddata", init_extractpic, false);
+            video.addEventListener("canplaythrough", init_extractpic, false);
         }