[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
The job of this method is to identify various error conditions and throw the appropriate error message. The code should show clearly what those error conditions are and how they are determined.