Add and remove customers from an order with GraphQL

The Admin GraphQL API now includes orderCustomerRemove (to remove a customer from an order) and orderCustomerSet (to add a customer to an order) mutations, making it easier to manage orders and customers. The orderCustomerRemove mutation covers the same use case as setting order.customer: null in the REST API, while orderCustomerSet is all-new functionality not previously possible in GraphQL or REST.

We've also added an Order.number field, equivalent to the order_number REST field.