Add link to fleet dashboard

What does this MR do and why?

Add link to fleet dashboard

This change adds a link to the fleet dashboard when users have it enabled.

This change is still behind the runners_dashboard feature flag.

Screenshots or screen recordings

How to set up and validate locally

1. In rails console (rails c) enable the feature

   Feature.enable(:runners_dashboard)

2. Visit any the Admin -> Runners page http://gdk.test:3000/admin/runners

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 #390921 (closed)