Wrap List-Unsubscribe link in angle brackets

What does this MR do?

Fix the format of the List-Unsubscribe header.

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

Don't think so. I can't actually test this, though: Gmail only shows the link for senders with high reputation, which apparently doesn't include my local machine 🙂 iOS 10 Mail has this feature too, but it only appears to work for mailto links - same for Outlook.com. Any other ideas? 😕

Why was this MR needed?

Bug in new feature.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• 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?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/22453.