Pages: "Custom domains" plus "separate server" configs don't overwrite "Support for domains and certificates is disabled"

Summary

Custom pages domains can not currently be easily configured if a separate Pages server is used. Reported by a Premium customer (internal ticket).

Steps to reproduce

1. Setup a separate Pages server
2. Apply the custom domain config => A documentation update could clarify here that this needs to be on the GitLab server.
3. Try to configure a custom domain.

Example Project

What is the current bug behavior?

The Support for domains and certificates is disabled… message in a project's Settings->Pages is shown

What is the expected correct behavior?

The custom domain config should be shown, same as when the Pages daemon runs on the main GitLab server.

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

 (For installations with omnibus-gitlab package run and paste the output of: \`sudo gitlab-rake gitlab:env:info\`)  (For installations from source run and paste the output of: \`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production\`)  

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of: `sudo gitlab-rake gitlab:check SANITIZE=true`)

(For installations from source run and paste the output of: `sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true`)

(we will only investigate if the tests are passing)

Possible fixes

• Adding gitlab_pages['external_http'] = ['some.bogus.ip.fake:8181'] to the GitLab server's gitlab.rb, and documenting this work-around
• expanding app/views/projects/pages/show.html.haml#L21 to take into account that edge case of:
  • external.http/s NOT present, but
  • the 4 items from separate server step 11 BEING present

Implementation:

• GitLab: !190501 (merged)
• Omnibus: omnibus-gitlab!8419 (merged)
• Documentation: !192193 (merged)
• Charts: #549023 (closed)