[#13144] Implement 'Copy' button in course enroll page

[simonliuanu]

Fixes #13144

Outline of Solution Added a 'Copy' button in course enroll page to let instructors to import existing students into new students sheet below to support the feature proposed in #13144.

Before implementation: After implementation the new page looks like this with the 'Copy' button: After clicking 'Copy', students are copied with information shows up: