Dashboard: New panel in a dashboard is not deleted after "Discard"-ing changes in Panel Edit (#66476)

* add isNew notPersistedProperty to PanelModel

* if panel is newly created and user "Discard"s it, the panel is removed entirely

* add Todo's for when we remove the emptyDashboardPage FF

* add isNew to new panel after file dropping on dashboard page

* handle the "Apply" case

* CSV file dropping is not relevant to a new panel bc it doesnt open edit page

public/app/features/dashboard/state/DashboardModel.ts

@@ -293,6 +293,7 @@ export class DashboardModel implements TimeModel {
   }
 
   private getPanelSaveModels() {
+    // Todo: Remove panel.type === 'add-panel' when we remove the emptyDashboardPage toggle
     return this.panels
       .filter(
         (panel) =>