Improve loading experience of Task in a modal

Related to #360835 (closed)

Replace loading spinners for work item with skeleton loader.

Screenshots

Screen_Recording_2022-05-02_at_9.34.06_pm

How to test

1. Enable work_items feature flag

   Feature.enable(:work_items)

2. Visit an issue page, and add a checklist item to description
3. Hover over checklist item, then click 'Convert to task' button on the left
4. Save the work item
5. Click on the newly created work item
6. Skeleton loader instead of misaligned spinners

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.