#155 cut down on event and exhibition sidebar reloading

Route: `/sorted/<any>/by-current-and-upcoming` Sidebar loads and reloads posts anytime it appears. Very chatty. Payload is small <10kb but adds up and lots of extra backend calls. ## On pages with a sidebar, I want to: - Check for loaded upcoming events and - upcoming exhibitions before - asking the `wp-api` for new data and - see the upcoming events and exhibitions For example: TBD ## Acceptance criteria: - Every list and - every single with a sort has - a sidebar that checks for upcoming events and - upcoming exhibitions before - attempting to load more posts