Feature: change JSON cache structure to store linkbacks requests status

[Lucas-C]

This would allow to easily audit which URLs have been successfully submitted, and the error responses for the ones that haven't.

Suggested new JSON structure:

{
  $article_slug: {
    $url: {
      "pingback": { // optional, no pingback support detected if field is absent
        "server_uri": "http...",
        "submission_successful": boolean,
        "error": // optional string
      },
      "webmention": { // optional, no webmention support detected if field is absent
        "server_uri": "http...",
        "submission_successful": boolean,
        "error": // optional string
      }
    },
    ...
  },
  ...
}

This schema change must be implemented so that the old JSON caches can still be parsed, but are then saved with the new structure