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.