System will delete existing art_files of the same name/job/upload_type

This merge should come after #19.

This hides the "destroy" link for users who have a 'dnet' role, and tweaks the UI.  The coffeescript now updates the uploaded file's name (to show it's sanitized name) and removes existing table rows when files are "overwritten".

This also automatically overwrites files of the same *sanitized* name.  In milestone 2.0, we may want to change this so it renames the existing file on AWS and marks the art file record as "overwritten", if we are concerned about potentially losing art files, but it should be OK to start.

Tests are being written now and this will be merged once complete.