[FIX] website_slides: embed YouTube videos without cookies

Due to recent legal changes in EU, you cannot install tracking cookies in a user's browser without explicit consent.

Before this patch, Odoo embedded youtube videos were violating that law (AFAIK & IANAL).

With this simple patch, you benefit from YouTube's offering to embed videos without cookies. The UI remains the same, but no tracking is done.

@Tecnativa TT26153

closes odoo/odoo#61601

X-original-commit: cd23b3e7
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>