Add admin organizations route, controller, view, and menu item

What does this MR do and why?

Related to #419540 (closed)

The grouptenant scale team is starting to build out an Organization MVC. For more information about what an "Organization" will be see https://docs.gitlab.com/ee/architecture/blueprints/organization/

This MR setups the route, controller, view, and menu item for organizations view in the admin area. This view will allow admins to manage the organizations in their instance. The view will be rendered with Vue and using GraphQL as the data source. This will be implemented in a follow-up MR.

MR acceptance checklist

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Screenshots or screen recordings

How to set up and validate locally

1. Open Rails console
   • bin/rails console
2. Enable the feature flag
   • Feature.enable(:ui_for_organizations)
3. Sign in as an admin
4. Navigate to /admin/organizations