[IMP] sale_quotation_builder: uniformize back button

Before this commit, the back button was a small button top right.
But in other modules, the back button is a full width button, like on /my/order
This commit adapt the back button to have the same style by using the adequate
template.

To do so properly, the `o_portal_fullwidth_alert` div was moved from
`portal.portal_layout` to `portal.frontend_layout`.
That way, templates calling `website.layout` can also use it as it inherits
from `portal.frontend_layout`.

task-1943427

closes #31387

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


Co-authored-by: Mitali Patel <mpa@odoo.com>
Co-authored-by: Romain Derie <rde@odoo.com>