Add email template when source_user rejects reassignment

Justin Ho Tuan Duongrequested to merge
473974-user-mapping-email-group-owner-when-reassignment-is-rejected-frontend into master

What does this MR do and why?

Sent when an Import::SourceUser rejects a reassignment.

Changelog: changed

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Import_source_user_rejected_email Import_source_user_rejected_email_text

How to set up and validate locally

Similar to !157729 (merged).

Related to #473974 (closed)

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