Richtext editing for task descriptions

What does this MR do and why?

Add content editor for work item descriptions, behind feature flag

Screenshots or screen recordings

Using existing content editor component, so some differences from designs on #363007 (closed) (buttons at bottom). Adding support for that separately.

flag disabled	flag enabled

How to set up and validate locally

1. Enable work_items_mvc_2 feature flag
2. Create a task
3. Click task title to open modal
4. Edit the description, use richtext editor

MR acceptance checklist

• I have evaluated the MR acceptance checklist for this MR.

Related to #363007 (closed)