Vanity URLs to articles are being returned to Absorber

[seanmtracey]

Exists in #6

The feed generated by this app returns FT vanity URLs, which are not accepted as valid by the Absorber, which only accepts ft.com URLs with UUIDs

//Example:
  <item>
    <title><![CDATA[May confirms intention to form new government despite losses]]></title>
    <link><![CDATA[http://localhost:4567/audio.mp3?narrator-id=Amy&uuid=af661888-c095-33da-8c7a-da1f8b719863&is-human=false&format=mp3&hashcode=4cd4cb94769201b7757c8d98faf6800d&duration=60]]>
    </link>
  <description><![CDATA[deliberately left blank]]></description>
  <pubDate>2017-06-09T09:16:27.000Z</pubDate>
  <guid isPermaLink="true"><![CDATA[http://www.ft.com/fastft/2017/06/09/may-confirms-intention-to-form-new-government-despite-losses/]]></guid>
</item>

Though not explicitely stated in the spec (my bad), the guid parameter needs to be either an http://www.ft.com/cms/s/:UUID URL, or a https://www.ft.com/content/:UUID.

When one of the items fails, all of the items in that feed are rejected.