Notifications: Enable Notifications to be sent to "unassigned" users when a specific Task is completed

[#1]

Need the ability for a User to be notified when a specific Task is marked "Complete" without that User needing to be assigned to the following Task or receiving blanket "Project Task Completed" notifications.

This would be useful for clients who need to be alerted when a specific Project Task has been completed (e.g. data has been submitted to a vendor), but aren't necessarily assigned to Tasks in that Project.