Consider caching old common location set to check if we should rerender the truth plot, instead of using an observer

[nmdefries]

To save plot rendering/computation time in https://github.com/cmu-delphi/forecast-eval/pull/271, we could also save the previous locationsIntersect to a user session-specific global and check during the next call to summaryPlot() if it has changed. If locationsIntersect has changed, make a new truth plot.

Potential problems:

• Because it is in an observer, the re-plotting logic in https://github.com/cmu-delphi/forecast-eval/blob/49896afaece66985f0b0d59c3eab1dee20bfe32a/app/server.R#L239 is confined to cases where the forecaster selection has changed. If we move the logic to summaryPlot() truth re-plotting could be triggered in other cases that we don't need/want.
• It's unclear how much extra time making a new truth plot takes. The re-plotting logic re-renders the old plot and rendering is slow.