feat: add browser dialog to prevent navigation (openedx#962)

Switching from undefined to "" tells the browser to put in a modal which requires users to confirm thier navigation away. this will prevent continued annoyances from upload failures: https://2u-internal.atlassian.net/browse/TNL-11587

src/files-and-videos/videos-page/VideosPage.jsx

@@ -85,9 +85,12 @@ const VideosPage = ({
   useEffect(() => {
     window.onbeforeunload = () => {
       dispatch(markVideoUploadsInProgressAsFailed({ uploadingIdsRef, courseId }));
+      if (addVideoStatus === RequestStatus.IN_PROGRESS) {
+        return '';
+      }
       return undefined;
     };
-  }, []);
+  }, [addVideoStatus]);
 
   const {
     isVideoTranscriptEnabled,