Hide practice button if practice handler isn't provided
Hide practice button if practice handler isn't provided

Even though the PracticeButton doesn't use the onPractice callback, it's
a good way to determine if practicing sections is allowed.  The prop is
only given for the Student component, not Teacher or TeacherStudent.

openstax/tutor-js,openstax/tutor-js,openstax/tutor-js,openstax/tutor-js,openstax/tutor-js