Currently when a user is disabled, they continue to receive notifications, causing extra load on the server etc. I can't think of a reason why disabled users should receive notifications if they can't log in to change/delete them.

However, they should continue to be stored in case the user is only temporarily disabled - just the population of the notification tables should cease for that user.

Steps to reproduce

1. Create a new user
2. Log into the Admin interface as that user
3. Add a subtree notification for the user
4. Log out
5. Log in as Admin & change something under the subtree
6. Run the notifications cronjob.