[FIX] portal: restore BS3 clearing system for main frontend containers
[FIX] portal: restore BS3 clearing system for main frontend containers

BS3 used to add ::before and ::after elements on all containers so that
margins and floats are cleared inside containers. As lots of current
odoo layouts may rely on this for some alignments, this is restored (at
least for a while) here only for main containers of the frontend.

In master, we should use proper layouts to avoid the need of this.

task-1889238

closes odoo/odoo#30510
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo