Don't allow transition to paid if invoice was not successfully approved before

Purpose
Don't allow to pay for the invoice which was not approved before

Overview:
In current realization it is possible to pay for the invoice without approval and it leads us to situation, when voucher are not created (voucher are created only on Approval stage).

Requirements/Scope:

1. Block payment until invoice will be approved
2. Clear error should be returned

Approach:
Add validation in the org.folio.rest.impl.InvoiceHelper#updateInvoice:
1. Check that transition from "Approved" -> "Paid"
2. Check that "approvedBy" and "approvalDate" are filled.

Acceptance criteria:

• Logic updated
• Unit test created
• API tests created