Add a method that returns related items in order

Mongoid will preserve the order of related_artefact_ids. It does not respect
that ordering when using the relation (related_artefacts), and instead returns
them in natural order. The order of related_artefacts is semantically important
to us.
Merge pull request #53 from alphagov/ordered_related_artefacts

Add a method that returns related items in order