Fix classifications ordering

This branch fixes #5578 by doing the following:

* adds scope to `acts_as_list` in `Spree::Classification`;
* adds migration to rebuild all classification lists as scoped;
* alters the spec for product scopes to test for expected behaviour.

I'll be using your CI server to test against regressions, because on my machine the whole suite takes nearly 1h to finish. So excuse me if there will be several commits.