HTTPS only Pages

What does this MR do?

Add the option of forcing GitLab Pages HTTP traffic to HTTPS, as set out in https://gitlab.com/gitlab-org/gitlab-ce/issues/28857.

Dependent on: gitlab-pages!50 (merged). Marking as WIP while this remains open.

CI

All jobs passing except ee_compat_check. The conflicts look trivial, but I believe that I can't make this job pass without access to gitlab-ee repo?

Testing locally

To test locally you will need:

• GitLab Runner service active, with Docker executor available
• GitLab Pages server compiled from this branch and running

Happy to add more detailed steps if they would be helpful to anybody.

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

None known.

Why was this MR needed?

Closes #28857 (closed).

Screenshot

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

#28857 (closed)