Show comments threads in work item activity list

What does this MR do and why?

This MR focusses on showing the threads to a comment in work item activity

Screenshots or screen recordings

Collapsed threads	Expanded threads

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Enable the work_items_mvc flag

   Feature.enable(:work_items_mvc)

2. Visit any task and try writing comments and threads
3. Make sure the threads are visible

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.