Use JWT to Authorize CRUD Endpoint for User

I added a controller for CRUD operations on User objects. It uses JWT to authenticate and authorize the user. Read-only operations are open to everyone, but read-write operations are limited to admin users.

This was an opportunity to refactor app_api/routes/index.js to separate authentication-related endpoints, /api/register and /api/login, from the user-related endpoints and place each group in its own routing file.