Make Artefact#as_json handle missing tags gracefully

Previously this was blowing up if tag_ids referenced non-existent tags.
This changes it to just omit these tags from the resulting tags array.

This change is necessary to allow the JSON view of the panopticon artefacts index to work (it currently blows up on various pages).  This in turn is necessary to allow us to seed the url-arbiter with the data that's currently in panopticon.