[FIX] website_slides: add missing class to show discussion tab panel
[FIX] website_slides: add missing class to show discussion tab panel

Task #1923443

Purpose
=======

Probably due to the migration to bootstrap 4, the active tab panel in the 'slide_detail_view' was not displayed
on page loading (and only when switching back and forth in the tabs).

partly closes #29901
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo