[6.x] Improve collapsible section trigger

[jaygeorge]

Description of the Problem

Collapsible sections rely on a hover state to initiate the animation, which is a neat way to prevent animations from firing off on page load. However, it's quite easy to trigger the animation snapping to 0ms if you're speeding around the CP, and moving your cursor around quickly.

What this PR Does

• Adds a class to publish sections on first interaction. This is a small helper class, which means we don't need to rely on hover hacks
• I've also moved the CSS into a global stylesheet, since we use the very same CSS on the form builder—so we don't need to update it in multiple places anymore

How to Reproduce

1. Open/close collapsible sections in the publish form
2. Quickly pull your cursor out of the section while it's animating and observe the animation abruptly end