Support custom domains when pages run on separate server

What does this MR do and why?

Support custom domains when pages run on separate server

Related: #285089 (closed)

Changelog: fixed

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Create a GitLab Pages website from a project template, preferably using the plain HTML template.

2. In your GitLab repository, update the gitlab.yml file with the following settings under the pages section:

   ## GitLab Pages
   pages:
     external_http: false
     external_https: false
     custom_domains: https

3. Restart the Rails web server using:

   gdk restart rails-web

4. Verify the following under Deploy > Pages:

   1. The Force HTTPS (requires valid certificates) checkbox is visible.
   2. The New domain button is visible in the Domains section.
   3. On clicking the New domain button, the New pages domain form appears, containing Certificate (PEM) and Key (PEM) text areas.

MR acceptance checklist

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