Draft: Start adding description widget to work items

What does this MR do and why?

Add description widget to work item form and mutation

todo:

• autosave field (confirm that cancel deletes autosaved changes)
• border box and reduce width and layout
• render GFM on work item root page
• formatting buttons down the bottom(?)
• tests
• Description updates in real-time via GraphQL subscription
• edit description
• Preview changes

Screenshots or screen recordings

1. Enable the feature flag

   Feature.enable(:work_items)

2. Visit create work item page on a project, i.e. http://gdk.test:3000//gitlab-org/gitlab-shell/-/work_items/new
3. Describe your work item

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.