Migrate "Expand" MR sidebar button to ButtonComponent

What does this MR do and why?

This migrates the "Expand" MR sidebar button to the ButtonComponent ViewComponent. This results in a minor visual change as the angle-double-left icon now inherits the button's size, which is slightly bigger than the previous s12 icon size. Though, this does not warrant a changelog entry since the button is currently behind a disabled-by-default feature flag.

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable the moved_mr_sidebar feature flag.

   Feature.enable(:moved_mr_sidebar)

2. Navigate to any MR.
3. Make sure the viewport is narrow enough for the right sidebar to collapse.
   • The Expand button should be visible to the right of the tabs.

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 #363969 (closed)