Store key seed with secure enclave/keychain

[jurvis]

KeysManager takes in a 32-byte seed to derive keys. We generate one here but currently persist it trivially by writing it to disk unencrypted.

Instead, we should store it in the Keychain or, even better, the Secure Enclave. We likely will need to support both, for Macs without a touch bar, Touch ID, or Apple Silicon.