Begin to create a controller framework to facilitate an Organization admin area. This will allow existing admin controllers to be moved to Organizations later, and should enable us to reuse the current controllers and views to bring instance features to Organizations. This work may encompass: * Create an overarching `Admin::Organizations::ApplicationsController` * Forbid `Current.organization` use in the non-Organizations `Admin::ApplicationsController` * Document the admin controller framework in development documentation. Kamil started a [draft MR ](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/202226)for this a while back. Pick up that work and finish it. However, consider not moving any current controllers to Organizations yet. The first bits added to the Organization admin area should probably be editing the organization itself. Followed closely by user management. Those are out of scope for this issue.