WIP: Feature/graphql

[bricesanchez]

/!\ This is a Work In Progress, please don't merge it now but feel free to test it and report any bugs.

I suggest to use GraphQL as an official API for Refinery. It has a lot of benefits against REST, read this for more informations: https://help.shopify.com/api/storefront-api/graphql

[UPDATE 2018/09/11]

My goals are :

• [x] Refactor to use new Class name based API: http://graphql-ruby.org/schema/class_based_api
• [ ] Provide a complete API (Queries and Mutations) for the main project.
• [ ] Handle I18n
• [ ] Write logic for role/user abilities on this API
• [ ] Write a test suite for this API
• [ ] Configure it to be used with refinerycms-authentication-devise
• [ ] Configure it to be used with refinerycms-inquiries

@refinery/collaborators don't hesitate to review the code and to point me better practices.