Artefacts for needs

An initial stab at providing a way to get a list of artefacts claiming to solve a given need. I'm not entirely sure whether this should be its own method or a query parameter on /artefacts.

It also feels like there's probably a way to refactor the creation of the paginated sets to reduce some duplication, but I decided that was out of scope for this change.

Should probably be reviewed by either @fatbusinessman or @JordanHatch