I am not notified anymore when this particular artifact is updated

Given that I participated in the artifact in some way (comment, field update, etc)
And if I choose to unsuscribe to this particular artifact (not the whole tracker)
Then I am not notified anymore of the modifications on this artifact until I participate again to it

- We still have the [unsuscribe] link in the email notification
- On each artifact, the user have a checkbox "[x] notify me" next to [submit] button. By default it is checked (it keeps the current behavior) and every time I create or update the artifact the list of subscribers is changed accordingly to the state of this checkbox (I am removed or added). If someone put me in CC or in assigned to of an artifact, then I am added to the list of subscribers. This checkbox will be available during a masschange.