DB changes for order-invoice-relationships table

Overview

In order to make it easier to discover which invoices are related to an order, and to manage these relationships, we need to make a few changes to the order-invoice-relationships table.

• Add a unique constraint on (purchaseOrderId, invoiceId)
• Add alias/table alias to help with cross-index subqueries

Acceptance Criteria

• changes above are made
• data migration scripts ensure uniqueness, removing records as needed
• unit tests are updated