Assign related artefacts based on their slugs

to progressively enhance relating artefacts feature,
we allow content designers to enter slugs of artefacts
to relate on the artefacts form. this change help build
that field on the form.
Artefact field to fill in related artefact slugs

and related controller changes to make that work.
removing the helper that returns relatable items
as we no longer need it.
Temporarily removing relating artefacts using JS