Introduce a new "Allow immediate deletion" application setting (default: true)

1. New allow_immediate_namespaces_deletion application setting (default true)
   • Set it to false for GitLab.com and Dedicated (in a post-deploy migration for existing installations, production fixtures for new installations, and through model validation to ensure it cannot be enabled)
2. Add support to update it in the Admin
3. Replace disallow_immediate_deletion feature flag with this setting (and remove the feature flag), and update group/project settings page
4. See if the actions from group/project list pages need to be updated
5. Update documentation