Add container and thread options in MR sticky header

What does this MR do and why?

• Wraps header content in container (dependent on feat(css): Add fixed layout util (gitlab-ui!3008 - merged))

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable :moved_mr_sidebar
2. Scroll down on MR on both fluid layout and fixed layout preferences

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 #372143 (closed)

Related to ✨ Beautifying our UI: Create:Code Review ✨ (#364574 - closed)