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/pom.xml

@@ -24,6 +24,10 @@
 			<groupId>org.springframework.boot</groupId>
 			<artifactId>spring-boot-starter-thymeleaf</artifactId>
 		</dependency>
+		<dependency>
+			<groupId>org.springframework.boot</groupId>
+			<artifactId>spring-boot-devtools</artifactId>
+		</dependency>
 		<!-- Test -->
 		<dependency>
 			<groupId>org.springframework.boot</groupId>