Update the domain model based on latest discussions

* Tag is gone;
* Topic (for notes) and Context (for tasks) got reintroduced instead;
* Note class inheriting from Artifact got introduced (to avoid stupid
situations like a topic associated to a task);
* Artifact is now purely semantic construct, it has no queries or
repository of its own anymore;
* Tasks can only have other Tasks as child;
* Projects keep mixing Tasks and Notes (IOW Artifacts) just fine.