Adding config variable so CMS can do uploads without needing to install ImageMagick

Added a config variable (skip_thumbnails) which makes CMS bypass the thumbnail processor and the Identify call in Paperclip that require ImageMagick to be installed. This allows Comfortable Mexican Sofa to run with full file uploads  and the least amount of dependencies (no ImageMagick), especially if image manipulation is not the point of the developer using CMS.