Last created plot is reused on dashboard refresh

Make some selections in the dashboard, e.g. change the forecaster selections and as-of date, and let the truth plot render. Reload the dashboard. Although the user selections will have reset, the truth plot is the same as the last one shown before reloading. This must be a bug in the [reuse-last-truth-plot code, e.g.](https://github.com/cmu-delphi/forecast-eval/blob/dev/app/server.R#L341-L347).