Datadog integration only available at project level

What does this MR do?

Related to #284088 (comment 543080333). Datadog integration is under the datadog_ci_integration feature flag controlled by project. At the moment, I only see 38 projects with the integration enabled.

But even the integration is disabled at the project level, it's available at the instance level (admin of the GitLab instance) and group level (group admin). Since the feature flag is restricted by project, we should move the integration as an integration only available at the project level.