[FIX] website_slides: fix multiple course review bug

Before this commit, it was possible for one to post
multiple reviews when all the already posted reviews
had the 'Employee Only' state. A traceback was also
thrown when trying to update an 'Employee Only'
comment. It was also possible to edit a log note.
This commit fixes all these issues.

Task-2810085

closes odoo/odoo#120570

X-original-commit: c3b11554
Signed-off-by: Stéphane Debauche (std) <std@odoo.com>
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>