Fix unapproved unassigned merge request e-mails failing

What does this MR do?

CE backport, https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18007

• Fix unapproved unassigned merge request e-mails failing to send
• Move approval email specs to EE specific directory
• Add email preview for unapproved unassigned merge requests

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

I noticed this while working on https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5031

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• 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 #3092 (closed)