Add group level work items route

What does this MR do and why?

This MR adds a new group level work items route, at /groups/<group_full_path>/-/work_items.

Subsequent MRs will add a frontend Vue app onto this route.

No changelog as this is behind the feature flag namespace_level_work_items

Issue: #392812 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:namespace_level_work_items)

2. Visit any group work items path http://127.0.0.1:3000/groups/flightjs/-/work_items
3. Confirm it loads a webpage

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 #392812 (closed)