Geo: Support GitLab Pages access control on secondaries

Problem

After https://gitlab.com/gitlab-org/gitlab-ee/issues/9337 or gitlab-pages#195 (closed), Pages access control would not work out of the box.

Possible solution

• Have secondary GitLab Pages use the primary for OAuth.

Acceptance criteria

• An admin can enable Pages access control on a primary and secondary
• When Pages access control is enabled, a logged-out user can view a public Pages site
• When Pages access control is enabled, a logged-out user cannot view a private Pages site
• When Pages access control is enabled, a project member can view a private Pages site
• When Pages access control is enabled, and a private Pages site was turned public, an anonymous user can view it
• When Pages access control is enabled, and a public Pages site was turned private, an anonymous user cannot view it
• A developer can find documentation for how to get this working
• Open any follow up issues