Split invoice feature

[flack]

Reported by flack on 28 Aug 1977 01:24 UTC Invoices created by the cron service will always collect all billable items on one invoice. There might be situations where users want to split this into multiple invoices, which ATM can only be done manually in Asgard. Technically, it's not too complicated to implement this, the question is only what the GUI should look like

Migrated-From: http://trac.openpsa2.org/ticket/153