feature/BankTransferEdit Fixed several issues with bank transfer:

- Ensure that get_bank_trans_for_bank_account returns all non void
  transactions.
- check_bank_account_history can now handle a 'balance_offset' representing
  the change to the balance that a void would make.
- check_bank_account_history now returns the correct amount when
  the proposed delta results in a negative balance in the account.
- check_bank_transfer now explicitly considers the four accounts
  affected by the transfer.
- Fixed the validation of new transfers and edited transfers.