Download selected submissions as archive, for targeted correction.

def downloadArchiveAction(self, request, queryset):
        '''
        Download selected submissions as archive, for targeted correction.
        '''
        output = io.BytesIO()
        z = zipfile.ZipFile(output, 'w')

        for sub in queryset:
            sub.add_to_zipfile(z)

        z.close()
        # go back to start in ZIP file so that Django can deliver it
        output.seek(0)
        response = HttpResponse(
            output, content_type="application/x-zip-compressed")
        response['Content-Disposition'] = 'attachment; filename=submissions.zip'
        return response