Add 'GET item' endpoint

[wilsonpage]

When a client renders a URLs it need to know everything about them. This includes data from Magnet Content Service and metadata from the web.

Improves magnet-client

Right now the magnet-client has to make two separate requests and merge the results to create the data-model needed to render an item view. This risks leading to inconsistencies.

Improves magnet-metadata-service

magnet-metadata-service has recently introduced a dependency on content-service to resolve extra_metadata and call_to_action. If we had this new endpoint we could remove this coupling and leave magnet-metadata-service to focus on only on extracting metadata from web pages.

---

Ideally we want to POST a list of urls and receive back a list (of matching length and order) something like:

{
  // url that uniquely identifies this item within magnet-service
  "id": ...,
  "lat": ...,
  "lon": ...,

  // merge of web metadata and magnet-content-service metadata
  "metadata": {
    ...
  }
}

[wilsonpage]

@samgiles @arcturus to comment.