Use message source in Web UI sample

Update the sample application to show how a message source can be
used in the view tier.

Closes gh-14947
See gh-14882

spring-boot-samples/spring-boot-sample-web-ui/src/main/java/sample/web/ui/mvc/MessageController.java

@@ -64,7 +64,7 @@ public class MessageController {
 			return new ModelAndView("messages/form", "formErrors", result.getAllErrors());
 		}
 		message = this.messageRepository.save(message);
-		redirect.addFlashAttribute("globalMessage", "Successfully created a new message");
+		redirect.addFlashAttribute("globalMessage", "view.success");
 		return new ModelAndView("redirect:/{message.id}", "message.id", message.getId());
 	}