Add instance level enable/disable for Secure Files

To align with other features that support object storage backends, the Secure Files feature should support the ability to fully enable/disable the feature on the instance level via the GitLab configuration. See https://docs.gitlab.com/ee/administration/job_artifacts.html for an example.

Expected behavior:

1. This feature should remain enabled by default
2. When disabled, the Secure Files section in CI/CD project settings will not be shown
3. When disabled, all API requests should respond with 403 Forbidden