Merge branch '28781-pages-namespaces-virtual-domain' into 'master'

app/models/namespace.rb

app/models/pages/lookup_path.rb

app/models/pages/virtual_domain.rb

app/models/pages_domain.rb

app/models/project.rb

app/models/project_pages_metadatum.rb

app/services/projects/update_pages_service.rb

...logs/unreleased/28781-pages-namespaces-virtual-domain.yml

db/migrate/20190909045845_create_project_pages_metadata.rb

db/schema.rb

lib/api/internal/pages.rb

spec/lib/gitlab/import_export/all_models.yml

spec/models/namespace_spec.rb

spec/models/pages/lookup_path_spec.rb

spec/models/pages/virtual_domain_spec.rb

spec/models/pages_domain_spec.rb

spec/models/project_spec.rb

spec/requests/api/internal/pages_spec.rb

spec/services/projects/update_pages_service_spec.rb