Send e-mail on CSV reassignment completion

What does this MR do and why?

Send e-mail on CSV reassignment completion

This MR introduces an email summary to be sent when CSV-based bulk placeholder reassignment completes.

References

• #458841 (closed)

Screenshots or screen recordings

How to set up and validate locally

Check the mailer previews

• bulk_import_csv_user_mapping_success
• bulk_import_csv_user_mapping_failed

Testing in GDK

• Follow the reproduction steps from !160748 (merged), but include invalid usernames/email addresses for some rows.
• Check letter_opener and inspect the sent mails.