Private Key Backup: Add support for Mnemonic keys for backup / restore

[rabble]

Manyverse presents user keys using a BIP39 Mnemonic instead of the key hash that we're currently displaying. In order to let users move their accounts back and forth we should add support reading and writing identities from bip39.

https://github.com/pengpengliu/BIP39

[storyaddict]

Thank you for opening this. I've opened a corresponding issue with Manyverse. It would be great to get this worked out between you both. :)

https://github.com/staltz/manyverse/issues/80

[rabble]

@Chardot is working on design in progress #251

[xloem]

Thank you for valuing this feature.

staltz/manyverse#80

For completeness, it looks like the manyverse side is awaiting a contributer to forward port and finish it at https://gitlab.com/staltz/manyverse/-/issues/1746 .