[FIX] website_slides: fix undeletable question answer

Issue

	- Go on Runbot V13
	- eLearning
	- In a course, try to delete a question

	You can't

Cause

	The linked answer is not deleted
	If you want to delete the answer you should
	add an extra menu

Solution

	Add ondelete='cascade' on answer's question_id
	Prevent course deletion if there are attendees

OPW-2210096

closes odoo/odoo#48372

X-original-commit: 799ee72facbca7a7eaf348938ed312a6d723854e
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/website_slides/models/slide_slide.py

@@ -457,6 +457,11 @@ class Slide(models.Model):
         rec.sequence = 0
         return rec
 
+    def unlink(self):
+        if self.question_ids and self.channel_id.channel_partner_ids:
+            raise UserError(_("People already took this quiz. To keep course progression it should not be deleted."))
+        super(Slide, self).unlink()
+
     # ---------------------------------------------------------
     # Mail/Rating
     # ---------------------------------------------------------