Adapt Ci::Runners::RegisterRunnerService service to take application setting in consideration

In #386712 (closed), we added settings to the app and namespaces to determine whether the runner registration token would be allowed at instance, namespace, and ultimately, project level. We now need to update the Ci::Runners::RegisterRunnerService service to take those settings into account, as mentioned in https://docs.gitlab.com/ee/architecture/blueprints/runner_tokens/#details.