Rethink how we're calling the Shopify API

[arobbins]

Right now, we're calling our WP REST API endpoints which then make the real calls to Shopify. We made this decision to improve the security of the Storefront API keys.

However, there are major reasons why we should make these calls in JS directly:

1. Takes the load off the WordPress sever, when many customers are buying at once
2. Allows for faster load times by removing a middle-man server call