[FIX] website_forum: allow new user to see own pending post

How to reproduce the bug:
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- Create a new user
- Log in as that user
- Go to a forum that don't require karma to ask a question.
- Create a new post
- On validation, you'll end up on a 404 page.

If we use the portal user instead, we see the post with a banner explaining
it has to be validated. This should be the result here too.

Specifications:
--------------

- When new users (karma = 0) create a post, redirect them to the page
relating to their question.

task-3372016

closes odoo/odoo#128382

Signed-off-by: Stéphane Debauche (std) <std@odoo.com>