Revert "Disable editing of related artefacts."

This reverts commit ad991d97bc22f7bd7bc38683977942c0c4bbca59.
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
Merge pull request #160 from alphagov/enable-relating-artefacts

Enable relating artefacts