MythNews: improve media:content parsing in rss feeds

* use the medium or type attributes to determine whether the content is an
  image or video. Previously we ignored the type and ended up trying to play
  an image.

* if we haven't found any thumbnail image then use any images found in the
  media:content as the thumbnail. Fixes #11161.

* parse all the media:content items not just the first one we find.