Reliable message back-up and recovery

nephros on OpenRepos:

Yes, reliably backing up the data would be very useful, I have repeatedly lost media and conversation history through reinstall/re-registrering.

• Create an extra binary (e.g. whisperfish-backup) that generates back-ups and that can restore the data. Especially useful for easier testing from community members.
• Integrate back-up into app itself.
• Integrate back-up into SailfishOS back-up
• Don't export the linked recipient id's into the backups (we can't assume the device recipient id's don't change between devices/backups/restores)