Simplify issuable list markup

What does this MR do and why?

This MR simplifies issuable list markup so that there's some fewer DOM nodes and CSS classes

Screenshots or screen recordings

No visual changes

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. In rails console enable the issues page refactor feature flag

   Feature.enable(:vue_issues_list)

2. Visit any project issues page, e.g. http://127.0.0.1:3000/h5bp/html5-boilerplate/-/issues
3. Check the code changes

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.