Changes to the callback that updates an edition whenever an Artefact is saved

This changes the `update_editions` Artefact callback so that it does less. This is in the spirit of reducing the coupling between the Artefact and Edition classes. The main issue behind these changes is to prevent errors occurring when saving an artefact would attempt to edit a published edition, raising an exception.

The changes are:
- After an artefact is saved, only update the `slug` for the edition.
- Only update the editions when the artefact is in draft state.