Fix assignees widget on group-level work item

What does this MR do and why?

This MR fixes the assignees widget to also work for group-level work items (#428037 (closed))

The errors on the group work item page are expected as we are still fixing backend/frontend problems to get work items at the group-level. This is part of viewing work items at the group-level (#393240 (closed))

Behind feature flag namespace_level_work_items

Screenshots or screen recordings

Before	After
Screen_Recording_2023-10-16_at_1.10.59_pm	Screen_Recording_2023-10-16_at_1.11.20_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. Click on created epic and confirm you can add/change the assignees

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 #428037 (closed)