Newest first isn't sorting system notes consistently

What does this MR do and why?

Fix work item notes when sorting by newest first. Change notesArray to a computed property, and just reverse the notes as a last step. Amend the writeQuery when adding a new comment to always add to the end (as list is then reversed).

How to set up and validate locally

1. Enable work_items_mvc feature flag
2. Add a task to an issue
3. Add some comments to the task
4. Sort the comments by Newest first
5. Refresh the page

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 #389177 (closed)