[FIX] website_forum: Fixed maximum recursion depth

Before this commit, you can set the question of a post like the post itself.
That will generate a traceback:
    RecursionError: maximum recursion depth exceeded in comparison

Now we check that there is no recursion

addons/website_forum/models/forum.py

@@ -427,6 +427,11 @@ class Post(models.Model):
                 raise KarmaError('User karma not sufficient to post an image or link.')
         return content
 
+    @api.constrains('parent_id')
+    def _check_parent_id(self):
+        if not self._check_recursion():
+            raise ValidationError(_('You cannot create recursive forum posts.'))
+
     @api.model
     def create(self, vals):
         if 'content' in vals and vals.get('forum_id'):