Send notifications through the Elgg 1.9 notifications system.

Comment tracker notifications are now sent asynchronously through the Elgg 1.9 notifications system.

The plan was to change the way how personal notification settings are stored (as metadata instead of relationships), but this implementation uses the old way for now.

This PR does not yet remove the old notifications code. I will remove it in a separate PR.

Tested and good to merge. (The remaining TODOs are not blockers.)