Add work items due date widget

What does this MR do and why?

This MR adds the work items due date widget with real-time updates, using the UI mockups in !93204 (comment 1082944527).

Behind feature flag work_items and work_items_mvc_2 defaulted to off, so no changelog required.

Screenshots or screen recordings

mobile	desktop
Screen_Recording_2022-09-12_at_3.03.29_pm	Screen_Recording_2022-09-12_at_3.05.32_pm

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:work_items)
   Feature.enable(:work_items_mvc_2)

2. In an issue, create a task in the Task widget then click on it
3. Test the due date widget in the task modal

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 #365399 (closed)