Integrate work item weight widget with backend

What does this MR do and why?

This MR integrates the work item weight widget with a real backend query and mutation for reading and updating.

Behind feature flags work_items and work_items_mvc_2, so no changelog required.

Screenshots or screen recordings

Screen_Recording_2022-07-19_at_1.56.36_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. Edit an issue's description to create some tasks, then save
3. Hover over a task, then click on the Create task + icon
4. In the modal, change the weight
5. Refresh the page and confirm the weight has persisted

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 #362550 (closed)