Speed up sidebar transition

What does this MR do and why?

What

• Removes $sidebar-transition-duration in favor of $gl-transition-duration-medium
  • Cuts the transition time by a third

Why

• The current transition speed adds to perceived sluggishness of the entire site
• Our many, many fixed elements need to shift around with the sidebar, and the two shift at very different speeds (they still do, but it's less noticeable now)

Screenshots or screen recordings

(These gifs don't really reflect what it looks like when you try it locally 🤷)

Before	After

How to set up and validate locally

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

Expand/collapse any sidebar

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