Project invoicing should support partial payments

**Reported by flack on 26 Nov 1918 08:37 UTC**
ATM, when invoicing closed tasks with a lower sum than marked in the deliverable, the deliverable will be adjusted. There should be an option to skip the adjustment, so that you can create multiple invoices for the same tasks as partial payments

Migrated-From: http://trac.openpsa2.org/ticket/145
