Deselect all sections at once

After opening the "Show X Results" dropdown, I'd like to be able to deselect all of the options with one click. That way if there's many sections, you can choose the few you want without clicking on 10 of them to deselect them.

Start by locating the implementation of the “Show X Results” dropdown and its section-selection controls. Check how option changes are handled, then add a single action that clears all selected sections and verify that users can reselect only the sections they want.

Written by the indexing model from the issue text.