Fix "Unsubscribe" link in notification emails that is triggered by anti-virus

What does this MR do?

• The unsubscribe link in an email body only unsubscribes automatically when logged in, otherwise the user is asked for a confirmation.
• The unsubscribe link in an email header unsubscribes automatically whether logged in or not.

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

This addresses all the comments from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5241, I think!

Why was this MR needed?

People were getting unsubscribed automatically by AV software.

Screenshot

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Fixes #17198 (closed).