Huge amount of work to (mostly) allow .ogg (and maybe other) formats to skip transcode

Huge amount of work to (mostly) allow .ogg (and maybe other) formats to skip transcode

 - Update get_display_media in several ways:
   - now uses the media type's own declaration of the order of things
   - returns both the media_size and the media_path, as per the docstring
   - implicitly uses self.media_files as opposed to forcing you to pass it in
 - update videos to use get_display_media
 - update images to declare media_fetch_order in the media manager (videos also)
 - update stl to use media.media_files['original'] instead of weird
   use of get_display_media
 - update sidebar to only conditionally show webm_640

TODO still: identify video type information *during* processing, show
that in the <video><source /></video> element.

This commit sponsored by Nathan Yergler.  Thanks, nyergler!