On save, only update slug for edition from artefact

As the section, department and business_proposition fields are 
deprecated, there’s not a good reason to denormalize these to an edition
whenever an artefact is saved.

The canonical source of the name of an edition is Edition#title, not 
Artefact#name [1] so it doesn’t make sense to denormalize this back 
again either.

[1] https://github.com/alphagov/govuk_content_api/blob/bf9ea854/lib/presenters/minimal_artefact_presenter.rb#L24
Only update edition slug when draft artefacts are saved

The only time when a slug can be changed for an artefact is when its in
draft, therefore we only need to update the edition for a draft artefact.

In addition, attemping to edit many fields on a published edition raises
an error as they cannot be edited. This will prevent these from
happening.
Remove redundant test.

There’s been a TODO notice to remove this test for more than two years
now, and I don’t think this test is needed anymore given the other
changes in this branch.
Make test descriptions a bit better
Merge pull request #170 from alphagov/artefact-edition-update-callback

Changes to the callback that updates an edition whenever an Artefact is saved