add contact recovery secret

[mixmasala]

each contact should have a "recovery phrase" used to re-rendezvous with other contacts, if for example a user device is lost along with the client state.
this should be exportable and printable, e.g. as a QR code or word-phrase salad that can be backed up on paper

there needs to be a privacy preserving mechanism so that clients can discover when their contacts request a re-rendezvous.