feat: restore seed from bip39 seed phrase

[holzeis]

I started working on that since its so hard to get to testnet money. It's getting tricky though with the lightning wallet, as it's seed is also derived from the bip39 seed.

• [ ] reload wallet once seed has been restored.
• [ ] create backup of seed before overwriting it.
• [ ] reload lightning wallet once seed has been restored.
• [ ] only allow restore if no cfds exist.

resolves #384