[IMP] website: allow to have custom slug Ux

Now, slug uses seo_name if field exists before to fallback on display_name.

It allow to have a custom url without change the product name already used in
backend e.g. or just because you want add some keywords for seo.

task-2291676

closes odoo/odoo#54152

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>