Warn duplicates skipped and transfer only verified credits

[annagav]

What are the relevant tickets?

N/A

Description (What does it do?)

When we merge accounts:

1. We want it to print the duplicate enrollments
2. filter out grades only for verified enrollments.

How can this be tested?

Run the command
./manage.py transfer_user_course_records --from_email=old@example.com --to_email=new@example.com

makes sure only verified grades are transferred.

[github-actions]

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