Send published documents to the content store.

https://www.pivotaltracker.com/story/show/79283750

Does not include the full content of the documents as this is only for the purpose of emailing topic updates.

I've made an assumption here that editions where fact check was skipped should be considered minor updates.

Future changes as part of this work:
- Adding topic/browse page associations (https://www.pivotaltracker.com/story/show/79283660)

Future changes not part of this work:
- Migrating to full content store tag format
- Adding full body of content
- Fully implement content store functionality,
  removing "placeholder" status
It would be better to use the [existing `stub_default_publishing_api_put` helper](https://github.com/alphagov/gds-api-adapters/blob/master/lib/gds_api/test_helpers/publishing_api.rb#L18) to do this.  Saves baking knowledge of the publishing API routes into this app.