overhaul multi-device recipients synchronization

[arcxio]

QOL update for multiple computers using the same password store.

recipients are additionally tracked in the password store and can be indexed in a git repository (resolves #50)

rekey is seamlessly integrated and is triggered automatically when recipients change

contrib/pa-rekey is repurposed to rotate identities with a better UX (resolves #53)

identity and recipients files are independently created

Apple's Secure Enclave and TPM support