[FIX] purchase: source document of invoice
[FIX] purchase: source document of invoice

When a vendor bill is created, no source document is added
automatically. Moreover, since the field is hidden when empty, there is
no way for the user to fill it.

The fix fills in automatically the source document field from the list
of PO.

opw-667055

bplancher/odoo,hip-odoo/odoo,laslabs/odoo,microcom/odoo,hip-odoo/odoo,dfang/odoo,stephen144/odoo,Elico-Corp/odoo_OCB,storm-computers/odoo,dfang/odoo,laslabs/odoo,stephen144/odoo,ygol/odoo,dfang/odoo,microcom/odoo,dfang/odoo,stephen144/odoo,hip-odoo/odoo,hip-odoo/odoo,storm-computers/odoo,bplancher/odoo,microcom/odoo,sysadminmatmoz/OCB,storm-computers/odoo,Elico-Corp/odoo_OCB,dfang/odoo,ygol/odoo,laslabs/odoo,dfang/odoo,microcom/odoo,laslabs/odoo,microcom/odoo,sysadminmatmoz/OCB,bplancher/odoo,sysadminmatmoz/OCB,ygol/odoo,Elico-Corp/odoo_OCB,Elico-Corp/odoo_OCB,bplancher/odoo,hip-odoo/odoo,ygol/odoo,laslabs/odoo,ygol/odoo,storm-computers/odoo,laslabs/odoo,Elico-Corp/odoo_OCB,hip-odoo/odoo,microcom/odoo,ygol/odoo,stephen144/odoo,sysadminmatmoz/OCB,storm-computers/odoo,stephen144/odoo,sysadminmatmoz/OCB,ygol/odoo,bplancher/odoo,storm-computers/odoo,sysadminmatmoz/OCB,sysadminmatmoz/OCB,stephen144/odoo,Elico-Corp/odoo_OCB,bplancher/odoo