Setup organization general settings route and navigation

What does this MR do and why?

Issue: #419075 (closed)
Navigation approval request: #417778 (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 adds the /settings/general route and navigation item. This page will be built out with Vue and GraphQL/REST APIs in future MRs.

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. Navigate to /-/organizations/default/settings/general

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.