FIX: correct notification when tag or category is added (#8801)

Regression was created here:
https://github.com/discourse/discourse/pull/8750

When tag or category is added and the user is watching that category/tag
we changed notification type to `edited` instead of `new post`.

However, the logic here should be a little bit more sophisticated.

If the user has already seen the post, notification should be `edited`.

However, when user hasn't yet seen post, notification should be "new
reply". The case for that is when for example topic is under private
category and set for publishing later. In that case, we modify an
existing topic, however, for a user, it is like a new post.

Discussion on meta:
https://meta.discourse.org/t/publication-of-timed-topics-dont-trigger-new-topic-notifications/139335/13

spec/models/topic_spec.rb

@@ -1400,6 +1400,27 @@ describe Topic do
               topic.change_category_to_id(new_category.id)
             end.to change { Notification.count }.by(2)
 
+            expect(Notification.where(
+              user_id: user.id,
+              topic_id: topic.id,
+              post_number: 1,
+              notification_type: Notification.types[:posted]
+            ).exists?).to eq(true)
+
+            expect(Notification.where(
+              user_id: another_user.id,
+              topic_id: topic.id,
+              post_number: 1,
+              notification_type: Notification.types[:watching_first_post]
+            ).exists?).to eq(true)
+          end
+
+          it 'should generate the modified notification for the topic if already seen' do
+            TopicUser.create!(topic_id: topic.id, highest_seen_post_number: topic.posts.first.post_number, user_id: user.id)
+            expect do
+              topic.change_category_to_id(new_category.id)
+            end.to change { Notification.count }.by(2)
+
             expect(Notification.where(
               user_id: user.id,
               topic_id: topic.id,