Use Section#uuid in PublishingAPIPublisher
Use Section#uuid in PublishingAPIPublisher

The introduction of the `entity_id` method isn't ideal but I think it's OK for
now. I'm imagining renaming `Manual#id` to `Manual#uuid` at some point in
future which will allow us to remove this method.

alphagov/manuals-publisher,alphagov/manuals-publisher,alphagov/manuals-publisher