Add namespace stats download button

What does this MR do and why?

It adds stats download button to the admin controls page.

Note: we can skip the UX review for now, the whole feature will undergo UX/TW review when it's ready.

Screenshots or screen recordings

Before	After

How to set up and validate locally

• In bin/rails console run:

    Feature.enable(:namespace_limits_admin_dashboard)
    ::Gitlab::CurrentSettings.update(check_namespace_plan: true)

• Simulate saas and restart your gdk

  $ export GITLAB_SIMULATE_SAAS=1
  $ gdk restart

• Visit /admin/namespace_limits as root

• Use the export button (you should see a flash message)

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 #422501