[FIX] account: invoice report, advances counted twice
[FIX] account: invoice report, advances counted twice

The below revision force the use of `ABS` on the
`price_subtotal_signed` because of a migration issue:
649b289fa10fcfd654a68ef6c923a07475f10296

This leads to issues regarding the advances, in
`out_invoice` invoices, where this amount
`price_subtotal_signed` is negative, and should
remain negative in this report, otherwise
the advance is counted twice: Once with
the advance invoice, and a second in the actual
invoice where the advance is removed with a line
with a negative `price_subtotal`

e.g.
 - A sale order of 500€ without taxes
 - Create an advance invoice for an advance payment of 10%: 50€ without taxes
 - A second invoice with the remaining: 450€ without taxes

In the invoices report, the total without taxes for this customer was
displayed as 600€ (50 + 500 + 50)
instead of   500€ (50 + 500 - 50)

opw-682390

ygol/odoo,bplancher/odoo,laslabs/odoo,bplancher/odoo,hip-odoo/odoo,bplancher/odoo,Elico-Corp/odoo_OCB,dfang/odoo,storm-computers/odoo,microcom/odoo,laslabs/odoo,dfang/odoo,Elico-Corp/odoo_OCB,bplancher/odoo,dfang/odoo,ygol/odoo,Elico-Corp/odoo_OCB,hip-odoo/odoo,hip-odoo/odoo,ygol/odoo,storm-computers/odoo,microcom/odoo,hip-odoo/odoo,laslabs/odoo,storm-computers/odoo,ygol/odoo,laslabs/odoo,bplancher/odoo,microcom/odoo,Elico-Corp/odoo_OCB,bplancher/odoo,dfang/odoo,dfang/odoo,hip-odoo/odoo,ygol/odoo,ygol/odoo,dfang/odoo,microcom/odoo,microcom/odoo,storm-computers/odoo,storm-computers/odoo,laslabs/odoo,microcom/odoo,laslabs/odoo,hip-odoo/odoo,Elico-Corp/odoo_OCB,storm-computers/odoo,ygol/odoo,Elico-Corp/odoo_OCB