Integrate email notifications into signup and login (#38)

frontend/app/test/ux_analytics_test.dart

@@ -131,7 +131,10 @@ void main() {
       );
       await tap(tester, find.byKey(const ValueKey('submit')));
       expect(find.text('Check your inbox.'), findsOneWidget);
-      expect(find.textContaining('We’ve sent you'), findsOneWidget);
+      expect(
+        find.textContaining('a verification email is on its way'),
+        findsOneWidget,
+      );
       await tap(tester, find.byKey(const ValueKey('demo-confirmation')));
       expect(find.text('Your email\nis verified.'), findsOneWidget);
       expect(sink.events.map((e) => e.journeyId).toSet().length, 1);