The GitLab Pages external-http and external-https arguments can be specified multiple times

What does this MR do?

Documents the common use case of multiple -listen-http and -listen-https arguments to GitLab Pages, and updates the GitLab CE code to cope with this reality.

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

Why was this MR needed?

Support for this is added in gitlab-pages!6 (merged) and omnibus-gitlab!1383 (merged). This MR needs to be merged before the omnibus MR.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• Tests
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together