Publishing docs generates a PublicationLog entry

When we publish a docuemnt generate a PublicationLog entry for use later
in generating change notes.
Export change notes to content-api

When we export documnts to the content-api we should also push the
change notes with it. Let thew exporter pull out the PublicationLog entries
and push them to the content-api.
Dont store publication_history on SpecialistDocumentEdition

We no longer need to store publication_history as this is now stored
in the PublicationLog. So we can remove this from the edition and export
it to thecontent-api on publish for use in the frontend.
Migrate change_history data from Editions
Test PublicationLogs are created
Merge pull request #336 from alphagov/feature/change-note-publication-logs

Change note publication logs