Add loading spinner for work item weight widget

What does this MR do and why?

Add a little spinner while weight is updating - from #429137 (closed)

Screen_Recording_2023-12-07_at_16.25.41

How to set up and validate locally

1. Enable work_items_mvc and work_items_mvc_2 feature flags
2. Load a task or an issue in work item view (from a regular issue, replacing "/issues/" in url with "/work_items/"
3. Change weight

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.