Align assignees labels inputs on small screens

What does this MR do and why?

Followup from #370283 (comment 1074856457)

Responsive fixes for work items

1. Move Labels and Assignees fields to same line as their label
2. Hide "Assign me" button on xs screens

before	after

How to set up and validate locally

1. Enable the feature flags

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

2. On an issue page, scroll below the description to the gray boxes section, click "Add" in the Tasks box
3. Enter a title, click Create task to view the task field
4. Check assignee and label field layout

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.