[bisq-network/bisq] Stop requesting traders private keys for Manual payout (#4061)

[huey735]

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### Description

<!-- brief description of the bug -->

At present, in the rare cases Mediators need to create manual payout transactions to release the funds from the multisig, they need to request the private keys of both traders and sign the transaction for them. This is a terrible violation of one of the core Bitcoin principles.

### Expected behaviour

<!--description of the expected behavior -->

Once a trader opens mediation the mediator gets access to the trade's contract with all the necessary information to create the transaction. They should do so and share the unsigned transaction to the traders so they can sign and broadcast it.

### Actual behaviour

<!-- explain what happened instead of the expected behaviour -->

The mediator creates. signs the transaction for both traders and broadcasts it.

### Screenshots

<!--Screenshots if gui related, drag and drop to add to the issue -->
![emergencyMultisigPayoutTool](https://user-images.githubusercontent.com/44140017/76680743-19059800-65e3-11ea-8a86-626bebbd565a.png)


#### Additional info

<!-- Additional information useful for debugging (e.g. logs) -->

I've created a guide for an [alternative method using tools outside of Bisq](https://bisq.wiki/Manual_payout). It'd be great if we could add the following functions to the Bisq software:
1. verify and sign message with a specific address
2. verify and sign a transaction
3. encrypt and decrypt message with a specific address

It's imperative that the sign, encrypt and decrypt functions are done inside the Bisq software due to the need of private key. Electrum and Coinbin's UI may be worth copying.

Related
[Legacy Manual payout process](https://docs.bisq.network/manual-dispute-payout.html)
Update to new protocol - https://github.com/bisq-network/bisq/pull/3694

ping @bisq-network/mediators @wiz @Bisq-knight 

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https://github.com/bisq-network/bisq/issues/4061
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