Add methods for changing user roles to index and User.class controller

1. change the path to "PUT /user" (PUT means update)
2. change the role change method in the frontend to make it's request to the new path and  let it send a json object as mentioned above
3. evaluate in the backend if the received json object contains an "ID" and an "role" attribute. If so, call the role change method. If not, return 400 HTTP Status code (400 means: Bad Request).