Namespace model should require organization

As a result of Organization contains namespaces (!120822 - merged) it's possible to create a Namespace and the Organization will default to the default organization at the database layer.

Eventually the Namespace object should be instantiated with an explicit Organization and we should not default to the default organization. The issue is to add a requirement that Namespace only save when organization has been defined.

It should be possible to instantiate the Organization by inferring from the current context. E.g.

• A child namespace will have the same organization as it's parent.
• A top level namespace will be created from within the organization level.

Status

File	MR
app/controllers/import/gitlab_groups_controller.rb	!161185 (merged)
app/controllers/ldap/omniauth_callbacks_controller.rb	!162170 (merged)
app/controllers/omniauth_callbacks_controller.rb	!162170 (merged)
app/controllers/registrations_controller.rb	!161598 (merged)
app/services/ci/runners/create_runner_service.rb	!161610 (merged)
app/services/groups/create_service.rb	!165533 (merged)
app/services/groups/update_service.rb	!165533 (merged)
app/services/users/create_service.rb	!165533 (merged)
ee/app/controllers/registrations/groups_controller.rb	!162059 (merged)
ee/app/services/gitlab_subscriptions/trials/create_service.rb	!165019 (merged)
ee/app/services/registrations/standard_namespace_create_service.rb	!162059 (merged)
ee/lib/ee/gitlab/scim/group/provisioning_service.rb	!164882 (merged)
ee/lib/ee/gitlab/scim/provisioning_service.rb	!164882 (merged)
ee/spec/controllers/ee/registrations_controller_spec.rb	!161598 (merged)
ee/spec/lib/gitlab/auth/ldap/user_spec.rb	!162170 (merged)
ee/spec/lib/gitlab/auth/oidc/user_spec.rb	!162170 (merged)
lib/gitlab/auth/o_auth/user.rb	!162170 (merged)
lib/gitlab/import/placeholder_user_creator.rb	!160756 (merged)
lib/gitlab/import_export/group/relation_tree_restorer.rb	!161185 (merged)
lib/gitlab/seeders/ci/runner/runner_fleet_seeder.rb	!160573 (merged)
lib/tasks/gitlab/update_templates.rake	!160585 (merged)
spec/lib/gitlab/auth/ldap/user_spec.rb	!162170 (merged)
spec/lib/gitlab/auth/o_auth/user_spec.rb	!162170 (merged)
spec/models/namespace_spec.rb	!165950 (merged)
spec/services/users/registrations_build_service_spec.rb	!161598 (merged)