Restore CSV import of contacts to a list

* Remove old attachment model (attachment.rb file was missing, strangely)
* Add file attachment directly to list model, with Paperclip
* Move import/upload_csv to lists controller (previously one was on users_controller, confusingly)
* Handle errors more gracefully (still a bit awkward how we're passing AR errors through, but it works).
* Basic tests for success, failure

Not using db_files, hopefully that's ok.