Actually handle chargebacks

[Changaco]

In #42 I implemented money quarantine to discourage fraud, but we need a few more things to actually handle chargebacks should they happen:

• modify cash_bundles to prevent disputed funds from being withdrawn even when they are past the standard quarantine period
• archive the information contained in cash_bundles before deleting them, so we can easily identify who withdrew the money from a fraudulent payin
• implement a function that uses these two improvements to locate and lock cash_bundles when a dispute begins
• add a "chargeback" transfer context
• implement a function that executes the chargeback if it is confirmed by the bank, by transferring all the recoverable funds to the LiberapayOrg account, and setting negative balances creating debts when there isn't enough money in the user's wallet
• modify payday to transfer money to the LiberapayOrg account when the intended recipient of the transfer has a negative balance settle debts

[Changaco]

We've just received our first chargeback, it's time to implement this.