Atom feed thumbnails

[maiki interi]

Not entirely on topic with `#658 </issues/658>`_ or
`#659 </issues/659>`_, but related.

My expectation was for the atom feed to contain the thumbnail of
the image (or equivalent of other media types). This is important
to me, as I want to widgetize/aggregate the feed on other sites.

I found two places where feeds are generated,
``mediagoblin/user_pages/views.py`` and
``mediagoblin/listings/views.py``:

::

        for entry in cursor:
            feed.add(entry.get('title'),
                entry.get('description_html'),
                content_type='html',
                author=request.matchdict['user'],
                updated=entry.get('created'),
                url=entry.url_for_self(request.urlgen))

I am not sure what the thumbnail is, but I am betting it is a
**summary** or maybe **content** (see the
`werkzeug atom docs <http://werkzeug.pocoo.org/docs/contrib/atom/>`_).
I think this is important, even if we are moving to make it a full
ActivityStream, since the federation I have in mind is something
akin to a dashboard view of new photos posted on my friends'
sites.

So, can we get a thumbnail in there? =]