Lineitem details should have a more direct association with invoice entries

[Tiffany Little]

There is a 1:many association between invoice entries and line items. One line item could potentially have many associated invoice entries. But really, it's line item *detail* where the real work happens--that's where the number of copies comes from, whether it's received or cancelled, what fund it's assigned to (and thus if it has a fund debit or not), etc.

But there's no real link between the invoice entry and the line item detail. We can track it back through the line item, but that unnecessarily obscures the link we need.

I also think it makes it more specific--*this* copy is invoiced on *this* invoice entry. Unlike line items with a 1:many association, line item details to invoice entry should be a 1:1 association. This becomes especially important when we get in partial shipments, and partial invoices.

My suggestion would be that when an invoice entry is created and saved, the line item details that it grabs should be stamped with a new field of "invoice_entry", the same as line item. If the invoice entry is detached, the value should clear.

Oh, and the lack of direct association can also lead to issues like bug 1933859.