[FIX] website{,_sale}_slides: fix join/buy links

This commit fixes the following fronted bugs:
  * One link to "buy course" 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

Task-2818136
Part of Task-2663320
FwdPort of odoo/odoo#88454

X-original-commit: 1a4368b3fce6206da555a1d04751b1aea53c6a29
Part-of: odoo/odoo#92104