Add ability to expand/collapse all search highlights

[eyeseast]

After #848, we're defaulting to showing search highlights by default, which can lead to highlights taking over the page. The old site shows the first three and allows you to show all (which adds state and complexity).

I propose we add a top-level toggle that expands or collapses all highlights in one action, similar to selecting all documents.

[allanlasser]

Top level toggle in ResultsList would work well, since we can pass in an open prop to the PageHighlight and NoteHighlight. Source

[allanlasser]

Two approaches to providing the controls:

1. Show a dropdown menu when there's an active search with actions, using a similar visual language to #866
2. In each group, show the "All" actions