Consider defining `USE_CURR_TRUTH` as a `reactive`

There are [several `observer`s](https://github.com/cmu-delphi/forecast-eval/blob/49896afaece66985f0b0d59c3eab1dee20bfe32a/app/server.R#L814-L820) that exist only to change the value of `USE_CURR_TRUTH`. The logic could be simpler and dashboard faster if `USE_CURR_TRUTH` were changed into a `reactive`. Since `reactive`s are lazily evaluated, they are faster than `observer`s.

Start in app/server.R around lines 814-820 and inspect the observers that update USE_CURR_TRUTH. Confirm how the dashboard currently responds to those changes, then verify that the reactive-based behavior preserves the existing dashboard results and improves the relevant update flow.

Written by the indexing model from the issue text.