[ Feature request ] Task observers

[lllopo]

Would be nice to be able to assign task observers. These would be users, that are attached to a task, but are not assigned to complete it. So - they will receive all notifications related to the task and they can track it's progress, but are not assigned to work on it. Same effect is achievable with just adding more users as assigned, but this way it is impossible to differentiate between observers and assignees.