Passing customer data through to AppointmentForm

The Git tag for this section is passing-data-through-loader.

In this section, we'll build an AppointmentForm so that it's ready for use in our new workflow. The main requirement here is that our App component can pass the customer prop through the AppointmentFormLoader and into AppointmentForm, and that the customer ID is submitted to the server when an appointment is saved. We'll split that work into two parts.

Passing through props to the child component

This requires two new tests, one in each of AppointmentFormLoader and AppointmentForm, and updating existing tests in AppointmentForm to provide a default value of the prop:

1. Add the following test in test/AppointmentFormLoader...