Add custom additional text to emails

What does this MR do?

CE backport, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18183

• Add custom additional text to emails

Are there points in the code the reviewer needs to double check?

• You can check the previews at http://192.168.1.135:3010/rails/mailers/devise_mailer
  • http://192.168.1.135:3010/rails/mailers/devise_mailer%2Fconfirmation_instructions_for_signup
• These previews are generated from spec/mailers/previews/devise_mailer_preview.rb

---

Emails layouts

• app/views/layouts/mailer.html.haml
  • app/views/layouts/_mailer.html.haml
• app/views/layouts/mailer.text.erb
• app/views/layouts/mailer/devise.html.haml
  • app/views/layouts/_mailer.html.haml
• app/views/layouts/mailer/devise.text.erb
• app/views/layouts/notify.html.haml
• app/views/layouts/notify.text.erb
• app/views/email_rejection_mailer/rejection.html.haml
• app/views/email_rejection_mailer/rejection.text.haml
• app/views/repository_check_mailer/notify.html.haml
• app/views/repository_check_mailer/notify.text.haml
• ...
• ee/app/views/layouts/service_desk.html.haml
• ee/app/views/notify/service_desk_new_note_email.text.erb
• ee/app/views/notify/service_desk_new_note_email.text.erb
• ee/app/views/notify/approved_merge_request_email.html.haml
• ee/app/views/notify/unapproved_merge_request_email.html.haml

Why was this MR needed?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #4474 (closed)