Always allow the need_id for an Artefact to be edited

Once over, Need IDs that looked like Need-o-tron needs could not be changed in Panopticon, for reasons unknown. When I added integration with Maslow needs, I kept the same behaviour for Maslow needs.

We'd now like to change this so that any Need ID can be changed for an artefact. This will make it easier for content designers to keep the artefact up to date with the most appropriate need (and to more easily correct any mistakes).

I've removed the `need_id_editable?` method from the Artefact model extension class. I've also changed the form so that the Need ID field is always shown (and editable) regardless of whether the Need API returns information or not.