Don't display events that existed before turning on notifications

Steps to reproduce:

1. Log in as Admin
2. Create a user e.g. U1
3. Create some pages and blog posts, delete and update others
4. Log in as U1
5. Go to Notifications Preferences
6. Set all notifications to ON
7. Refresh page

Expected results: There is no documentation to specify this behavior but, as a user, I am expecting to see only the notifications for pages/blog pots that are created after I set my notifications to ON. I don't expect to see any notifications for pages/blog posts created before that.

Actual results: All notifications for pages/blog posts that were changed from when the user was created appear.