I made a regression here 17366d3bcc (diff-ddeebb36d131f89ca91be9d04c2baefaR10)
When the tag is added, people watching specific tag are notified but also people watching specific category.
Therefore, `notify_post_users` should accept options who should be notified.
So when `category` is added to the topic, users watching topic and users watching category are notified.
When `tag` is added to the topic, users watching topic and users watching tag are notified
Finally, when a new post is created, everybody is notified, topic watchers, category watchers, tag watchers.
69266f60ed