Use permalink rather than slug in 2-1 and 2-0

My PR https://github.com/spree/spree/pull/4683 to `master` and `2-2-stable` turned out not to be backwards compatible with `2-1-stable` and `2-0-stable` due to the shift in nomenclature from `:permalink` to `:slug` that happened between 2.1 and 2.2.

This PR patches this and restores functionality to `2-1-stable` and `2-0-stable`.