Resolve "Setup organization routing for directory page"

What does this MR do and why?

This MR sets up the initial routing for Organizations, in the format:

localhost:3000/-/organizations/:organization_path/directory

As first step, hitting this URL, currently gives you an empty page with 200 OK. This page will be accessible only to instance admins, because we have not yet designed concrete access policies for organizations, as we currently do not have members for organizations.

This UI is behind a feature flag ui_for_organizations, since we do not want to expose this to administrators while development is ongoing.

Screenshots or screen recordings

How to set up and validate locally

1. Turn on feature flag via Rails console: Feature.enable(:ui_for_organizations)
2. Login as root user (who is admin)
3. Navigate to http://localhost:3000/-/organizations/default/directory, where default is the path of the default organization in gdk. Default organization is already seeded, so it should exist - you can see it using Organizations::Organization.first in Rails console.
4. You will be seeing an empty page, but you can add content to app/views/organizations/organizations/directory.html.haml, which will be reflected on the page after a refresh.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #409913 (closed)