Add edit mode to work item descriptions

What does this MR do and why?

Add edit button to Work item description. Related to #334809 (closed)

Use updateWorkItemWidgets mutation from !88955 (merged)

Screenshots or screen recordings

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:work_items)

2. Visit any issue page, add checklist item
3. on checklist item, hover, Convert to work item
4. Save, click task title to edit it
5. View & edit description

Known issues / limits

• currently doesn't handle conflicts at all (no lockVersion in mutation. Looking into this..)
• no autosave yet (may be followup MR)