Artefact has an archived? helper method

This method existed as a meta-programming hack within Panopticon. Apps
shouldn't have their own hacks on top of models and we should instead
keep this logic inside of the model.
Expose a simpler way of searching for related Artefacts

Relatable items shouldn't be archived, nor should they be completed
transactions. This is used in more than one place (Panopticon, Travel
Advice Publisher) so it's exposed a known method on the model.
Merge remote branch 'origin/add-pantopticon-artefact-enhancements-to-artefact-model'