Draw currency from better table on Batch Transaction form

Change from using civicrm_contribution.currency to using civicrm_financial_trxn.currency in the data displayed on the form to assign financial transactions to a batch (civicrm/batchtransaction?reset=1&bid=1). The currency is always related to an amount, and the amounts shown are from the civicrm_financial_trxn table, NOT the civicrm_contribution table.

This is relevant as the form is intended to allow financial data from other types of financial information as well eg grants, general journal entries. While these have entries in the civicrm_financial_trxn table, they don't have related civicrm_contributions, and a currency is still required.

Related IIDA ticket IIDA-109 which is dependent on IIDA-94