Configure a cache busting resource chain

This commit configures a resource chain with
`resources.chain.strategy.content.enabled`, enabling cache busting for
static resources based on the hash of their content.

A template helper (here a Groovy Closure) is added to the model given to
the view for appending hashes to static links such as:

    "/css/spring.css" -> "/css/spring-d35c4193cd32e7e44cda5737205c0c0e.css"

Fixes #321

initializr-service/src/main/groovy/io/spring/initializr/service/InitializrService.groovy

@@ -29,6 +29,7 @@ import org.springframework.context.annotation.Configuration
 import org.springframework.scheduling.annotation.AsyncConfigurerSupport
 import org.springframework.scheduling.annotation.EnableAsync
 import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor
+import org.springframework.web.servlet.resource.ResourceUrlProvider
 
 /**
  * Initializr service application. Enables legacy STS support for older
@@ -47,8 +48,9 @@ class InitializrService {
 	@Bean
 	@SuppressWarnings("deprecation")
 	LegacyStsController legacyStsController(InitializrMetadataProvider metadataProvider,
+											ResourceUrlProvider resourceUrlProvider,
 											GroovyTemplate groovyTemplate) {
-		new LegacyStsController(metadataProvider, groovyTemplate)
+		new LegacyStsController(metadataProvider, resourceUrlProvider, groovyTemplate)
 	}
 
 	@Configuration

initializr-service/src/main/resources/application.yml

@@ -13,6 +13,11 @@ spring:
   jackson:
     serialization:
      write-dates-as-timestamps: false
+  resources:
+    chain:
+      strategy:
+        content:
+          enabled: true
 
 initializr:
   env: