Incremental rollout for the Pages new API based config source

The ~"group::release management" team has been working on new API based config source for Pages (gitlab-org&1316 (closed)). This is now merged, deployed to staging, and on it's way to production.

By default the new config source is disabled and Pages is using the existing disk based one. We have implemented a "feature flag" like functionality to enable the gitlab config for specific domains until we are confident we ready to switch to it.

This is done by Pages watching a file. This file should be named .gitlab-source-config.yml and placed in the same dir the Pages process is running from, same as the .update file currently used. This should be Settings.pages.path which I think in the case of staging and production should be /var/opt/gitlab/gitlab-rails/shared/pages.

File content to start with (for staging):

domains:
  enabled:
    - vshushlin.staging.gitlab.io
    - staging.shushlin.dev
    - ka-public-test-group.staging.gitlab.io
  broken: non-existent-domain.staging.gitlab.io

We'll need help from someone with access to update this file when required on all Pages nodes.