Migrating eCommerce integrations to 2.4.0

Introduction

In 2.4.0, from the Integrator point of view, the most important change is a new checkout.js middleware - its main goal is to manage access to the checkout steps.

Changes

• Added checkout.js middleware that integrators have to implement to prevent access to checkout steps when previous steps were not yet completed.
• Refactor naming and signatures of some of core factory functions.
• Refactor boilerplate getters.