Add work items participant widget

What does this MR do and why?

Add work items participant widget (#432156 (closed))

Behind feature flag work_items_mvc

MR acceptance checklist

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Screenshots or screen recordings

Before	After	After (expanded)

How to set up and validate locally

1. In rails console enable the experiment fully

   Feature.enable(:work_items_mvc)
   Feature.enable(:work_items_mvc_2)

2. Visit any issue
3. Create a task from the Child Items widget
4. Open the task and confirm there is a participants widget

Related to #432156 (closed)