[IMP] website_[sale_]slides: allow users to take the slide quiz before joining the course

PURPOSE

When previewing a slide of type 'quiz', users are tempted to answer the
questions even if they are not member of the course yet.

This commit will allow users to answer the quiz without being a member of the
course, and offer a smart redirection to join / buy based on the enroll type
of the channel.

SPECIFICATIONS

We introduce a "CourseJoinWidget" that will handle the display and the behavior
of the "join" button based on the user (public or not) and the enroll type
(public course or based on a payment).

When the user answers questions and he's not a member yet, we store the answers
temporarily in the session object.
When he comes back on the quiz page after having completed his membership, we
fill in the quiz based on his previously entered answers and clean the session.

LINKS

Task 2045571
PR #41972

Co-authored-by: Florent Lejolyn <fle@odoo.com>
Co-authored-by: Aurélien Warnon <awa@odoo.com>