[IMP] website(_sale)_slides: Add login,join,buy button on quiz

This commit improves the elearning onboarding on the quizzes

If the user is not logged in, a log in button will show up under
the quiz

If he is logged in but not a member of the course and the course is public,
a join button will appear under the quiz

If the course needs to be bought, a buy button will appear under the quiz.

If the course is on invite only, a message under the quiz will inform the
user about it

TaskID: 2003986
PR: #33861

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