Feature Request: Support multi-sig accounts

[leighmcculloch]

What problem does your feature solve?

The same problem and idea that @tomerweller proposed here https://github.com/stellar/go/issues/432.

For multi-sig accounts that have a signer that isn't the master key of the account, and where the user doesn't have the master key anymore, there's no way to add an account and its signing key. I can add the signing key, but then web apps assume its G address is the account address, which is not the case.

What would you like to see?

1. The API expanded to distinguish getting the account public key, and the signer public key, which will be the same value for single key accounts, but will be different for accounts using a non-master key signer.

2. The UI expanded to support importing an account using only its signing key, where the extension looks up accounts on the network that have that signer (e.g. via https://horizon-testnet.stellar.org/accounts?signer=GAZUXDPWVSYD7HE33GZPCJW76QMAZTBEKLFUQCHO5SP2OOL52ANVYZCE) and offers to setup with one of them and the signer as the signing key.