Check for last backup age on startup

[jose1711]

Maybe your notification daemon is broken or you're in DND mode all the time, or perhaps just run the backups manually. For these situations it could be handy if vorta checks the age of your last backup (check is on by default) on startup and displays a warning if it reaches a configured threshold (e. g. 7 days w/o a backup). But maybe I'm just having paranoia combined with backup-OCD 😄

[m3nu]

Kinda implemented in #908, but no time to finish it yet.