Reimplement backup

[OffRange]

What should be included:

• Allow both one-time and periodic-auto (Auto backup #31) backups
• Option to select auto backup interval
• File formats: CSV, KDBX
• KDBX passphrase encrypted via key in KeyStore
• CSV: support generic headers/columns (import Microsoft Authenticator exported passwords using csv file. #32)
• Support non-local file destinations, e.g. NextCloud, etc