The standard use case for large scale migrations is usually to export the issue list using the csv report, then merge them and work from there.
While having a csv file per app is fine, we should offer the possibility to export (and download) an all-in-one csv - that should be the default handling when exportCSV is passed as parameter.

Not sure what would be the best option parameter-wise to allow the user to have a single file per app. suggestions welcome.

The location of the link which leads the user to the download should be displayed on the All Issues page of the report.