Move logic from exporter to PublicationLog

By encapsulating the logic for fetching change notes for a publication,
it's possible to reuse it with other exporters.
Merge pull request #227 from alphagov/refactor-manual-change-notes

Move logic from exporter to PublicationLog
Publish manuals to the Publishing API

The Publishing API will eventually replace the Content API.
Publish manual sections to the Publishing API
Publish manual change notes to the Publishing API

Change notes are added to the 'details' attribute on the 'manual'
content item, rather than being a separate content item.
Send manuals organisation info to Publishing API

The Publishing API has no concept of tagging just yet, so the organisation
association is stored in the 'details' attributes.