Add close button to work item modal

What does this MR do and why?

This MR:

• Adds a close button to the work item modal
• Moves the ellipsis button from next to the title to above the title

Behind feature flag work_items, default disabled. No changelog required

Resolves one of the tasks in #366210 (closed):

To allow work items to be closed on mobile, move the action menu three dot icon to a row above the title. When the work item is being rendered in a modal, show an X close action to the right of the three dot icon.

Screenshots or screen recordings

full page view	modal view

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:work_items)

2. Edit an issue's description to create some tasks, then save
3. Hover over a task, then click on the Create task + icon
4. Confirm the modal has a close button that works
5. Close the modal, then right-click on the task link and open in a new tab
6. Confirm the full page work item does not have a close 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.

Related to #366210 (closed)