build out retention reports

components/pages/reports/ReportDetails.js

@@ -1,9 +1,11 @@
 import FunnelReport from './funnel/FunnelReport';
 import EventDataReport from './event-data/EventDataReport';
+import RetentionReport from './retention/RetentionReport';
 
 const reports = {
   funnel: FunnelReport,
   'event-data': EventDataReport,
+  retention: RetentionReport,
 };
 
 export default function ReportDetails({ reportId, reportType }) {