[FIX] website_slides: fix resource ACLs

This commit fixes ACLs for slide resources:
 * Access to resources should be blocked for non-members (except publishers
 for the course: responsible and managers)
 * Tests are included

NB: As links are resources from this minor version on, their access
is protected as for files.

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

X-original-commit: 075a647d58dc7e817b31a80c2ac804b06c8492bb
Part-of: odoo/odoo#92104