Change author_account to be authors in REST API

[Gargron]

The author_name, author_url, and author_account properties on the preview card entity become:

{
  "authors": [
    {
      "name": "...",
      "url": "...",
      "account": null
    }
  ]
}

...to allow for multiple authors later on. author_name and author_url are kept for compatibility.

This change is breaking only within nightlies.

[renchap]

One small comment, otherwise good for me.

@ClearlyClaire can you have a look as well? And we will need to update the documentation with those (+ deprecate the previous author_name and author_url to encourage client authors to switch to the new one?)