Embed link data in the post

[annando]

See the discussion here: https://discourse.diasporafoundation.org/t/implementation-specific-fields/668/8 and here: https://discourse.diasporafoundation.org/t/enclose-the-oembed-data-in-the-status-message/1056/4

The idea behind is to control which link in a post is the most important, instead of choosing the first one to fetch OpenGraph/OEmbed data.

See the proposed structure:

<status_message>
  <author>alice@example.org</author>
  <guid>17418fb029e6013487743131731751e9</guid>
  <created_at>2016-07-11T22:38:19Z</created_at>
  <text>I've been totally shocked by the article that I just read!</text>
  <embed>
    <link>http://www.huffingtonpost.co.uk/2015/06/01/9-signs-cats-are-planning-world-domination_n_7482898.html</link>
    <title>Cats Are Conspiring To Take Over The World, And Here’s Proof</title>
    <description>It's fairly obvious to anyone who has been near a cat for a prolonged period of time that the little felines are clearly plotting to take over the world....</description>
    <image>http://i.huffpost.com/gen/3016630/images/o-CATS-PLOTTING-WORLD-DOMINATION-facebook.jpg</image>
  </embed>
  <public>true</public>
</status_message>

The "link" element is mandatory, the rest is optional. If all data is present, the system can rely on it and doesn't need to do a request on its own. Or it could do an OEmbed/OpenGraph request on the "link" anyway and could fall back to the provided data.