Reset brush when inputs change

[jwildfire]

Would be nice to remove brush when strata variable or statistic change. Should be doable with session$resetBrush, but it's non-trivial to use in modules. Seems like update will be easy once this PR is merged.

Once that's ready, we should be able to drop something like this in volcano_server:

observeEvent(input$stratification_values, {
     session$resetBrush(input$brush$plot_brush)
})