Communicate changes in pages configuration to self-managed customers

We're getting close to enabling the first part of the architectural changes on gitlab.com.

After we do that and see it works fine for some time, we want to make self-managed customers use it.

Open questions

1. When we'll delete support for current configuration by disk source?
   1. it will block us from delivering new features such us redirects support
2. How customers will enable new API source
   1. We can use the same method we used for .com(gitlab-com/gl-infra/production#1640 (closed))
   2. We can provide some omnibus switch
      1. We can make it on by default for new installations
3. How we'll communicate the breaking change to users and when
   1. We can communicate brake in 13.0, but actually remove support for disk source later(e.g. 13.2)