Switch to poltergeist for spree_backend.

Fixes #4383
Whitelist some params for canonical links. Fixes #4374
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.
Revert "Allow preferences to reference ENV variables."

This reverts commit e4a612c115ff2f03fea0943f2babde51a633cf8b.