fix(debit-invoice): calculate relative line items
fix(debit-invoice): calculate relative line items

Disabling the summing of previous line items for relative items broke
the default VAT included debit invoices.

It's now partially reverted in that we fall back to the old behaviour if
there is no saldo line item before the relative line item.

huerlisi/bookyt,huerlisi/bookyt,wtag/bookyt,huerlisi/bookyt,wtag/bookyt,wtag/bookyt