Fix showing discussions in group work items

What does this MR do and why?

Fix showing discussions in group work items

This is part of viewing work items at the group-level (#393240 (closed))

Behind feature flag namespace_level_work_items

Screenshots or screen recordings

Screen_Recording_2023-11-24_at_5.39.24_pm

How to set up and validate locally

1. In rails console

   Feature.enable(:namespace_level_work_items)

2. Visit any work items list page for a paid subscription group such as http://127.0.0.1:3000/groups/flightjs/-/work_items
3. Create an epic
4. Confirm you can view all discussions and can create comments

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 #428853 (closed)