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).

Reproduce the dashboard flow described in the issue, then inspect app/server.R around lines 341-347, where the reuse-last-truth-plot code is linked. Check how the truth plot behaves after a reload when the selections have reset; done means the previous truth plot is no longer reused after refresh.

Written by the indexing model from the issue text.