[FIX] mass_mailing: outlook.com need absolute url
[FIX] mass_mailing: outlook.com need absolute url

The `<base />` tag isn't handled by outlook.com, thus all relative link
to image fail to be shown.

This fix treat the most common case by using absolute url instead of
relative url.

related to opw-648463

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