[FIX] website_forum: convert answer to comment, use superuser

To browse the forum post, as the partner of the create_uid of the post is needed, which can't be read by portal users.

addons/website_forum/models/forum.py

@@ -420,7 +420,7 @@ class Post(osv.Model):
         """ Tools to convert an answer (forum.post) to a comment (mail.message).
         The original post is unlinked and a new comment is posted on the question
         using the post create_uid as the comment's author. """
-        post = self.browse(cr, uid, id, context=context)
+        post = self.browse(cr, SUPERUSER_ID, id, context=context)
         if not post.parent_id:
             return False