Simplify/Robustify thumbnail usage for template designers

[spaetz]

This is pre-work for the PDF media type handling which is supposed to show default pdf fallback icons in the beginning.

Please consider merging the branch get_thumb_url from my git repo at ​git://gitorious.org/~spaetz/mediagoblin/spaetz-mediagoblin.git.

Description of my change straight from the commit log:

    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.