[IMP] website_forum: better implementation of automatic nofollow attribu...

...te

on links in forum posts.

This is now done when creating / updating posts instead of updating the display
of the post. This means that old posts may contain nofollow attribute altough
the user has sufficient karma; however it is acceptable compared to the computation
we save by avoiding processing every post to display.

- Task: https://www.odoo.com/web?&debug=#id=10862&view_type=form&model=project.task&action=333&active_id=248
- Pad: https://pad.odoo.com/p/r.GNUisVzSLMg1K653