Add 'Pending' status for invoice and Commit money on transition to 'Open"

Overview: When an invoice is created with no reference to an order line or no reference to any encumbrances through order lines. There is no money committed for the value of the invoice until it is approved (as per UXPROD-2393). For some institutions this money must be committed earlier in the workflow.

Note: RM SIG discussion (May 1, 2020) revealed that we must commit money on approval at the very least, so awaiting payment values are accurate. Also, Transactions must have links to invoice(s) that contribute to awaiting payment value and this will be created as separate feature. Finally, for some institutions waiting until "Approved" to commit is later than desired so a separate feature will be created to capture work allowing a pending invoice state so Open invoice values can be committed.

Use cases:
Some institution prefer to consider money committed as soon as invoices arrive. This way as soon as invoices arrive the impact can be seen in the budget.
Institution uses approval step only to indicate that an invoice should be paid by the accounting system and when there are only one or two approvers it can result in a backlog of invoices for approval. In this case we want the money to show as committed earlier so staff can make more informed decisions about purchasing.

Requirements:

• Add a 'Pending' status for invoices
• Create awaiting payment transactions when invoice transitions from Pending to Open
• perform all necessary checks
• Allow user to continue to edit invoices that are Open and update awaiting payment values accordingly