[ML] Enable cell range selection. (elastic#18920)

This re-enables the feature flag to support selecting a range of cells in the anomaly explorer. Follow-ups to this PR will tackle limiting the allowed cells to select to avoid problems with too wide selections.

x-pack/plugins/ml/public/explorer/explorer_dashboard_service.js

@@ -17,7 +17,7 @@ const module = uiModules.get('apps/ml');
 import { listenerFactoryProvider } from 'plugins/ml/factories/listener_factory';
 
 module.service('mlExplorerDashboardService', function () {
-  this.allowCellRangeSelection = false;
+  this.allowCellRangeSelection = true;
 
   const listenerFactory = listenerFactoryProvider();
   const dragSelect = this.dragSelect = listenerFactory();