[FIX] website_slides: can join a course in a quiz preview

PURPOSE

Fix a bug where you get a traceback when trying to join a course on a quiz
preview not in fullscreen mode.

A variable was badly named in JS (id -> channelId), leading to the ID not
being forwarded to the "Join Course" sub-widget.

LINKS

Task ID 2079476
PR #38051

closes odoo/odoo#39876

X-original-commit: b32d30b8cde85ba37f9cef975838874a08ea7c07
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>