Bulk Data Entry postProcess needs to create entity_batch rows linking to financial_trxn rows (instead of contribution rows)

The Batch Export functionality requires that financial_trxn records get exported, NOT contribution records. This means we need to modify the insert into civicrm_entity_batch in the postProcess for Bulk Data Entry so it inserts a row linking to civicrm_financial_trxn for each contribution recorded:

Example: save bulk data entry for 2 contributions and save to Batch 3
As spec'd in CRM-11424 - we've created 2 contribution records and 2 financial_trxn records (let's say the financial_trxn row IDs are 55 and 56). New entries in entity_batch should look like this:

entity_table entity_id batch_id
-------------- ------------ -----------
civicrm_financial_trxn 55 3
civicrm_financial_trxn 56 3