Update clustered-highlights prototype to account for accessibility of "hidden" highlights

The clustered-highlights prototype (behind the `style_highlight_clusters` feature flag) allows one to change the colors used to style highlights clustered by ownership; it also allows you to "hide" clusters of annotations (shown here with "My annotations" hidden):

This was implemented in a quick-and-dirty way that technically just sets the background color for that cluster to `transparent`. Need to update such that screen readers do not read out "annotation start" and "annotation end" announcements in these hidden clusters or otherwise ensure that hidden highlights are sufficiently hidden for screen-reader users.