Fixes that calling e.g. `url_for @user.avatar` would return an incorrect URL resulting in a 404. The Active Storage URL helper ends up calling ActiveStorage::Attachment#signed_id, which previously delegated to ActiveStorage::Blob#signed_id but changed to return a signed *attachment* ID in 1a3dc42c.

The Active Storage controllers pass signed IDs to ActiveStorage::Blob.find_signed, so we need signed blob IDs.