mezzanine: filebrowser_safe does not work with Amazon S3 (and presumably other services of the same kind)

Trying to open the Media Library fails with the following exception when assets are hosted externally (in this case using django-storages on S3):

Stacktrace (most recent call last):
  File "mezzanine/pages/middleware.py", line 45, in process_view
    return view_func(request, *view_args, **view_kwargs)
  File "filebrowser_safe/views.py", line 76, in browse
    raise ImproperlyConfigured, _("Error finding Upload-Folder. Maybe it does not exist?")

This is a major issue when running Mezzanine on a PaaS like Heroku where persistent file system access is limited or completely unavailable.

The issue has already been discussed throughly here: https://groups.google.com/d/topic/mezzanine-users/kjsAw6WFqMY/discussion and here: https://github.com/sehmaschine/django-filebrowser/issues/40

There is already a fix in the upstream version of django-filwbrowser, although some other minor issues with thumbnail generation still remain.

If the fix cannot easily be incorporated in the existing fork, maybe it’s time to consider abandoning the *_safe-forks and use the newer versions of grappelli and filebrowser instead. Doing so would have the added benefit of

• Not having to maintain these packages separately and instead focus on the core of Mezzanine,
• Maybe fix some existing issues (I’m thinking #62)
• Gaining access to the progress and other improvements made since forking. Grappelli is looking really good now…

Keep up the good work!