[FIX] website: avoid duplication of cookie policy page

Before this commit a cookie policy page was created every time the
website settings were saved with the "Cookies Bar" option enabled.

After this commit the policy page is created only if it does not already
exist for the website.

task-2454424
https://github.com/odoo/odoo/pull/67217



closes odoo/odoo#67217

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