Handle AWS S3 Re-Authentication via s3.getSignedUrlPromise

AWS credentials have a limited lifespan.  If the credentials
have expired when you call `s3.getSignedUrl`, the call will fail,
although it will eventually succeed after the credentials are
refreshed in the background.

Calling `s3.getSignedUrlPromise` causes it to wait for the
credentials to refresh and then successfully return a signed url.

apps/meteor/app/file-upload/ufs/AmazonS3/server.ts

@@ -80,7 +80,7 @@ class AmazonS3Store extends UploadFS.Store {
 				ResponseContentDisposition: `${forceDownload ? 'attachment' : 'inline'}; filename="${encodeURI(file.name || '')}"`,
 			};
 
-			return s3.getSignedUrl('getObject', params);
+			return s3.getSignedUrlPromise('getObject', params);
 		};
 
 		/**