[FW][MERGE][FIX] website_{,sale_}slides: fix join/buy links and resource access

This commit fixes
* One link that was not updated to use the post_link class
* The join course link for "payment" courses after completing the quiz once
  logged in.
* The link to resources that became accessible for non-authorized users for
  "payment" courses

See underlying commits for details.

Task-2818136
Part of Task-2663320

closes odoo/odoo#92104

Forward-port-of: odoo/odoo#92055
Forward-port-of: odoo/odoo#88454
Signed-off-by: Warnon Aurélien (awa) <awa@odoo.com>
Signed-off-by: Florian Charlier (flch) <flch@odoo.com>