Search result 'Highlight all matches' should be a button instead of a link without a reference

The element to `highlight all matches` has an action on the page, it expands information to show all the matches.
This is typically an action that is done by a button. Links should be reserved for movement action: changing page or changing place in the page. If the action has an effect on the page, it should be a button.

This button should also probably have `aria-expanded = false` and `aria-control` set from the get go, so that AT users can leverage the semantics and use it with ease.