Refactor to separate "assigned item" component

What does this MR do and why?

This change moves the template that renders an assigned group of a runner to a separate "assigned item" component that can be reused.

Screenshots or screen recordings

No changes expected, this is the related component:

How to set up and validate locally

1. With some group runners added (https://docs.gitlab.com/ee/ci/runners/runners_scope.html#group-runners)

2. In rails console ($ rails c) enable the the view runner page

   Feature.enable(:runner_read_only_admin_view)

3. Visit Admin -> Runners

4. Visit any Group Runner page such as http://gdk.test:3000/admin/runners/108

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 #347856 (closed)