- Discount are the resource (in discount module)
- Trade Condition contains Discount Model Line
- Delivery contains Discount Line

(also apply some minor fixes to Tax* forms)

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@20621 20353a03-c40f-0410-a6d1-a30d3c3de9de