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](https://github.com/alphagov/govuk_content_models/pull/221)).

instead, we should infer an edition's section from its artefact. this causes a bunch of `n+1`s, which increases the response time. on the other hand, we're saving on the effort to keep section synced across edition and artefacts. with the versions of mongoid and mongo we're at there's no way to avoid `n+1`s. mongoid v3.0 introduces `.includes` with the use of identity map.

also, includes a migration to unset section field on existing editions.