Use section on artefact, edition section's deleted

www.agileplannerapp.com/boards/173808/cards/5444

in order to reduce coupling between Publisher and
Panopticon, we're removing the editions section
field (govuk_content_models v20.0.0).

hence, we should infer an editions section from
the artefact's section.

also includes a migration to unset section field
on existing Editions.
Merge pull request #249 from alphagov/use-section-from-artefact

Use section on artefact, edition section's deleted