Show order state changes

Allows an admin to view all state changes to an order from the order edit page. All state changes are already logged in Spree, but there's no way to view them.
I would prefer these used active record attributes instead. As an example: https://github.com/spree/spree/blob/87a9afd4a9f17b4b1ff73cae866ffd5a4ab25fec/core/config/locales/en.yml#L4-L13

Then you can use `Spree::StateChange.human_attribute_name(:type)` here. And a bunch of other stuff works nicer (like ActiveRecord validations).