display_name fixes

Fixing the way the display_name field is handled

- [x] change label to avoid mismatch
- [x] remove inverse method
- [x] remove search method
- [x] name_get should not use display_name, display_name should use name_get (breaks old api)
- [x] apply same to name_create
- [x] remove field display_name in forum as now in the orm
- [x] go green runbot !