Don't try to renumber existing attachments

Put new attachments in at -1 to avoid having to modify existing
attachment records in a specific order.
Set attachment ordering before save

Using before_add on the association in Attachable missed the case where
multiple attachments are added in one go. Each of the new attachments
would have the same ordering value. By using Attachment#before_save
instead the ordering values will be set just in time, so they can be
sequential.