Handling chargebacks / api support for different financial_type for refund

Background: 

A chargeback is initiated by the card holder and can (but doesn't have to) result in a return of funds. It's the bank's duty to determine whether such a request was reasonable.

A refund is a payment operation initiated by the merchant, it refers to a specific card transaction and allows to return the whole or part of the transaction amount.

This issue deals with adding support for chargebacks so they can be tracked separately.

Spec as agreed

1) Add new contribution status in 4.7.1 or 4.7.2 "Charge Back"
2) Enabled the Account Relationship 'Credit/Contra Account is' (renamed to Credit/Contra Income Account is) in the option group with name = 'account_relationship'
3) Add another Value to that option group 'ChargeBack Account is'
4) Update contribution BAO code so that when a contribution is updated to have status of Refunded or Chargeback the Financial Account that the reversals will be credited against (financial_item, financial_trxn) is the appropriate one of the above entries if it exists, failing that 'Income Account Is' will be used (no change)
5) Add appropriate unit tests
6) Notify 'money team'

Note that sales tax treatment will be unchanged - ie. where the revenue account was being credited this will become the Credit account or Chargeback account if configured - but the line items of any sales accounts and specifically the financial account for the reversals of the taxes won't be touched.

Also note that the new option values will be added using SQL that will not fail if they are already failed