Allow course CRUD based on user role

[kunalvarma05]

Use this for enrollment

enroll(course_id, student_id) {
  return new Promise((resolve, reject) => {
      let studentRef = userService.doc(student_id);
      this.find(course_id, ['students']).then((course) => {
        let courseStudents = course.students;

        if (!courseStudents || !courseStudents.length) {
          courseStudents = [];
        }

        courseStudents.push(studentRef);

        course.students = courseStudents;

        this.update(course_id, course).then((course) => {
          resolve(course);
        });
      });
    });
}