Wallet backup reminders

[luke-jr]

It'd be nice to get wallet backup reminders.

Seems some users forget to backup their wallet.

Even just a one-time message after creation would be a step forward, and/or we can have a counter that tracks non-backedup changes and reminds the user after some time.

This will also help ensure users are making backups correctly and not simply copying raw wallet.dat files.

Ideally, the message should explain how to update the backup in the future (without a reminder).

[jonatack]

Concept ACK

[Bosch-0]

Concept ACK. This could be applied in a spaced repetition fashion as to not give users too many notifications

[kristapsk]

Concept ACK