Mark voucher as paid on transition to invoice status "Paid"

Overview:
When an invoice transitions from "Approved" to "Paid", this should trigger a simliar transition in the corresponding voucher.

• When invoice.status => "Paid"
• Then the associated voucher.status => "Paid"

Acceptance Criteria:

• Voucher status transitions are triggered upon invoice transition as described above
• Unit tests are updated
• Api tests are updated