Add Notifications for Lessons when new content is available

See the umbrella Jira for this issue: https://sakaiproject.atlassian.net/browse/SAK-50034

The Lessons tool should generate notifications when new content is available.

Currently, the Lessons tool sends notifications when there are Comments or Replies to Comments on Lessons pages. However, since Lessons pages and items can also be selectively released by date or other criteria, it might be nice to also get a notification when there are new content items available for a user to access. Also, it would be a nice heads-up for students to know if the instructor adds a new module or unit to the course.

We would probably want to limit the notifications by page. For example “Lessons: Week 5 is now available” would be fine, but you don’t want to receive separate notifications for 15 individual items all on the same “Week 5” page.