The role permissions on group artefacts are stored in the artefact
object, so we might as well make use of that in the can_edit_artefact
and can_publish_artefact methods of the User class instead of forcing
a new query.

Change-Id: I7e5e5ff8e14fc55638a613a9253eb0c35fd60867
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>