Send Change History to Content API

With the change notes, we were only persisting the latest change note.
This commit inserts a hash of the timestamp and change note into a
change history Array and sends this to the content API.

It also deals with minor update checkbox and major update change note
persistence between versions.
Persist state of existing chnage notes
Update change history array on Publish

Now that we persist the change notes between updates, we don't want to
update the change history array on update. Instead we move this to
the Publish service.
Merge pull request #335 from alphagov/send-change-history

Send Change History to Content API