[FIX] portal, sale: scroll correctly if fixed header

Depending of the configuration, on the portal, anchor do not consider the
header, so a part of the text is hidden by the navbar.

task-2290213

closes odoo/odoo#60408

X-original-commit: 2cd9a9d12f4cd7352430c166145a7488af0c120c
Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>
Signed-off-by: Romain Derie <rdeodoo@users.noreply.github.com>