[FIX] website: parse url before redirect when langue updated

Before this commit, you can provide arbitrary url for redirect.
Now, we always return a relative url, since this method should be only called
from a page on your current website, their are no reason to let redirect to
another domain.

closes odoo/odoo#72848

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