Show unused amount of RTP in invoice allocation rows

[lcreid]

Show unused amount. Possibly show total RTP request. TBD.

[lcreid]

Currently shows an "Amount Available" column, but the number displayed appears to be the amount requested on the RTP (for the appropriate Part A or Part B (TBC)).

Suggest:

"Amount of Request": Display the amount requested on the RTP, for Part A if the InvoiceAllocation is for ServiceProvider, or Part B if the InvoiceAllocation is for Supplier. (This will have to be reviewed when we do the form that amends a request.)

"Amount Available": Display the Amount Requested, minus the amount already allocated by this and other invoices.

[lcreid]

The two columns are now displayed. The Amount of Request column is showing the right stuff. The Amount Available column is calculated on the back end, and is tested superficially by an integration test.

[lcreid]

Consider better unit or other tests.