Allow to disable `pages:root` in `config/gitlab.yml`

Since Pages can run in a fully remote type of access we should allow disabling a disk type of access for Pages as defined in Rails.

The FF is a good temporary thing to disable a NFS type of access, but we should rather have a proper behaviour implemented as a first-class configuration.

Maybe we change it to support:

pages:
    enabled: false
    access_control: false
    # The location where pages are stored (default: shared/pages).
    path: false

This would be the kill switch that would be used to ensure that we never touch a disk storage, if defined.

This also is how GitLab would be configured on GitLab.com: it would have path: false and object_store: configured only.