Simplify/Robustify the thumbnail URL usage in templates

So far templates required a very complex blurb to simply insert a
thumbnail URL, exposing much of the internal logic to the template
designer. In addition, we would fail with an error if for some
reason the media_files['thumb'] entry was never populated.

This adds the MediaEntry.thumb_url property that template designers
can simply use. It will do the right thing, either fetching the proper
thumbnail or hand back a generic icon specified in a media's
MEDIA_MANAGER as "default_thumb".

Add an image default fallback icon (stolen from Tangos, which are
Public Domain since version 0.8.90 as I understand) since the one
we referred to was not existing. Perhaps, a "broken image" icon
would be better, but I'll leave that to our capable designers.

All templates have been modified to make use of the new thumb_url
function.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

mediagoblin/media_types/image/__init__.py

@@ -24,5 +24,5 @@ MEDIA_MANAGER = {
                                 # 'mediagoblin.media_types.image.processing'?
     "sniff_handler": sniff_handler,
     "display_template": "mediagoblin/media_displays/image.html",
-    "default_thumb": "images/media_thumbs/image.jpg",
+    "default_thumb": "images/media_thumbs/image.png",
     "accepted_extensions": ["jpg", "jpeg", "png", "gif", "tiff"]}