Download-Button

[maede97]

When an export is started, but the user navigates off the tab, the export does finish. However, the download is not automatically started (at least for me). We should therefore also display a button called "Download", for example located underneath the status log. And we must be sure to only delete the data if they have either be downloaded automatically (how to detect?) or if the user pressed the button.
Or, even better, we should only delete downloads after a certain amount of time (e.g. 5 minutes?)