Fix filenames of App Backup Configs

[robi]

When downloading the backup configuration of an App, it always has the same name no matter the app, version, date, etc, which makes it really difficult to discern which is which on the local filesystem download folder.

Here's three different apps.

cloudron_app_backup.json
cloudron_app_backup(1).json
cloudron_app_backup (1).json

Please make them more unique and obvious.

[girish]

Thanks, fixed.