Delete section field on editions

www.agileplannerapp.com/boards/173808/cards/5444

there's this bug reported in Publisher where someone's adding a section to an artefact in Panopticon, yet the section is not showing up in the documents table in Publisher. this is because the editor adds `edition.artefact.section`, whereas Publisher shows the `edition.section` field. these 2 fields don't get synced after an edition is created from artefact metadata.

here's a proposal to delete the `edition.section` field altogether, so that we reduce coupling between these applications and don't have to worry about duplicating these fields across editions and their artefacts.

it seems that the Publisher documents table is the only place we use `edition.section`. all the more reason to switch that to use section from the artefact, and drop the `edition.section`.

taking an example of https://www.gov.uk/payslips. this edition has a section: "Work:Employment rights" its artefact has section: "Working, jobs and pensions:Your pay, tax and the National Minimum Wage". on frontend, the artefact's section is used to create breadcrumbs and report Section variable to Google Analytics. only Publisher shows the `edition.section`.