Use private instead of protected

See Dom's comments here:
https://github.com/alphagov/specialist-publisher/pull/181#discussion_r15332303
Rename attachment_services to services
Rename redirect_path to edit_path
Get repository dependency from wiring directly
Create AbstractAttachmentServiceRegistry
Refactor object type from service methods

We are already dealing with the AttachmentServiceRegistry so we don't need to call 'new_attachment'. Instead just use 'new'