Display 'Check namespace plan' checkbox in admin area on GitLab.com, staging, or in development

Merged Douwe Maan requested to merge dm-check-namespace-plan-checkbox into master

We need this in a 9.5 patch because we want to enable this on GitLab.com on September 1st (see https://gitlab.com/gitlab-org/gitlab-ee/issues/2392).

/cc @mydigitalself