Better alignment of collapsible arrows when there's a subtitle

[afercia]

As noted in #676 the collapsible down/up arrows should be slightly better aligned when the collapsible has a sub-title:

note about the arrow icons on the right: difficult to align them vertically when there's a subtitle, ideally they should be shifted a few pixels to the bottom

Unsure about the best way to handle this, as it depends also on the actual font-size. I'd tend to think the component should only be responsible to make the containing app aware there's a subtitle. CSS adjustments should go in the containing app.

[afercia]

I guess it would be nice to improve the icons position when there's a sub-title also because when entering a focus keyword, there's a visible "jumping" of the score icon on the left and the arrow icon on the right:

[afercia]

See https://github.com/Yoast/yoast-components/issues/716 Gutenberg has changed the icons for the sidebaar panels