Merge branch...

Merge branch '488584-jmontal-add-sm-onboarding-email-on-duo-enterprise-seat-assignment' into 'master' 

Add mailer for duo enterprise seat assignment

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/170507



Merged-by: Illya Klymov <iklymov@gitlab.com>
Approved-by: Roy Liu <rliu@gitlab.com>
Approved-by: Marina Mosti <mmosti@gitlab.com>
Approved-by: Heinrich Lee Yu <heinrich@gitlab.com>
Approved-by: Illya Klymov <iklymov@gitlab.com>
Reviewed-by: Marina Mosti <mmosti@gitlab.com>
Co-authored-by: Jay Montal <jmontal@gitlab.com>