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/containers/DashboardPage.tsx

@@ -351,6 +351,7 @@ export class UnthemedDashboardPage extends PureComponent<Props, State> {
     return updateStatePageNavFromProps(props, updatedState);
   }
 
+  // Todo: Remove this when we remove the emptyDashboardPage toggle
   onAddPanel = () => {
     const { dashboard } = this.props;