Implement Spree::Base as ActiveRecord::Base wrapper.

Moving inclusion of Spree::Preferences::Preferable away from an engine-base inclusion to ActiveRecord::Base, and instead making it a Spree-specific Spree::Base inclusion.  This is to prevent the Prefereble module from bleeding outward and causing conflicts in other Engines which may have attributes called 'preferred' on their models already

Fixes #4337
Fix superclass mismatch error.
Allow preferences to reference ENV variables.