Task description conflict resolution

What does this MR do and why?

Show warning if task description changes while you are editing it.

When description widget is updated while you are editing, assigns the updated description to conflictedDescription and displays then updated contents in a readonly textarea.

See #362967 (closed)

Screenshots or screen recordings

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How to set up and validate locally

1. Create a work item https://docs.gitlab.com/ee/user/tasks.html#create-a-task
2. Edit description of it, but don't save
3. Edit description in another window and save
4. See warning above Save button

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.