Revise media file storage

Problem:
Currently, resized/cached media files are stored in "some default folder" and this may not be the best idea.

Reading suggestions:
http://makandracards.com/makandra/16999-common-mistakes-when-storing-file-uploads-with-rails
http://www.therailsway.com/2009/3/16/storing-your-files

Task:
Ensure that media files are not kept for "too long" and that they "survive restarts".
