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!

mediagoblin/media_types/image/__init__.py

@@ -25,4 +25,8 @@ MEDIA_MANAGER = {
     "sniff_handler": sniff_handler,
     "display_template": "mediagoblin/media_displays/image.html",
     "default_thumb": "images/media_thumbs/image.png",
-    "accepted_extensions": ["jpg", "jpeg", "png", "gif", "tiff"]}
+    "accepted_extensions": ["jpg", "jpeg", "png", "gif", "tiff"],
+
+    # Used by the media_entry.get_display_media method
+    "media_fetch_order": [u'medium', u'original', u'thumb'],
+}

mediagoblin/media_types/video/__init__.py

@@ -26,4 +26,8 @@ MEDIA_MANAGER = {
     "display_template": "mediagoblin/media_displays/video.html",
     "default_thumb": "images/media_thumbs/video.jpg",
     "accepted_extensions": [
-        "mp4", "mov", "webm", "avi", "3gp", "3gpp", "mkv", "ogv", "m4v"]}
+        "mp4", "mov", "webm", "avi", "3gp", "3gpp", "mkv", "ogv", "m4v"],
+
+    # Used by the media_entry.get_display_media method
+    "media_fetch_order": [u'webm_640', u'original'],
+}