After add this constraint, some scenarios are not possible. So we
updated those related code and rspec together:
- Add not null constraint to gitlab_subscriptions namespace_id
- Remove always-true code logic `hosted?` from GitlabSubscription model
- Remove `context 'when namespace is absent'` since it is not possible
- Update several rspec file, to make sure create(:gitlab_subscription)
  will always have a namespace associated. So it won't break the
  not null constraint of gitlab_subscription's namespace_id.