[generic] return image files as `file` rel instead of `image`

This is to avoid presenting it as `photo` type in `oembed` endpoints
[Domains] video.pandodaily.com - no more

they moved to YouTube channel instead
[Facebook] Better post embeds.

* html5 embed code
* proper URL scheme
* include mobile links
* return 403 if post is private
* clean up 2 year old code for image parsers
[generic] allow XHTML mime to make FB mobile links work