[new][based on reverted project]admin can send join link to new instructors

1. redo the changes to the reverted project
2. now by default sample data will be imported for new instructor
3. modify the joinCourseForInstructor method to handle both instructors added by admin and other instructors
make this method private. Then, have two public methods, one with two parameters, one with three. Both public methods can call the private method. One can pass null as the third parameter.