Address small performance, accessibility and UI issues of the pinned items section

The following discussion from !113292 (merged) should be addressed:

• @pgascouvaillancourt started a discussion: (+1 comment)

  nitpick: Should this be a button?

  also it would be nice to add some aria attributes like aria-controls, aria-expanded

• @pgascouvaillancourt started a discussion:

  nitpick(non-blocking): Just for the sake of consistency, I would have preferred that we used borders, like we have in other parts of the super sidebar. But this should be fine too 👌

• @ohoral started a discussion: (+1 comment)

  same here about creating function in the template. Just extract it to the method

• @ohoral started a discussion:

  same performance issue - move this and next callback to the method