Add /management endpoint provided via Spring Actuator

The new endpoint provide metrics including information about application environment, configuration, beans, caches via Spring Actuator. Additional custom endpoints for version info and statistics are available. The endpoint is protected by Spring Security and requires the admin role.

To allow monitoring of metrics, a Prometheus endpoint is provided via Micrometer.

The /management/health endpoint times out at the moment but this is caused by existing issues with beans for external services.

Web API documentation: https://docs.spring.io/spring-boot/docs/2.1.x/actuator-api/html/

Known issue:
Content type application/json may not be set for the Accept header of requests to the management API. Otherwise, only a JSON stub is returned. application/vnd.spring-boot.actuator.v2+json or */* should be used instead.

Refs #16 (closed).

src/main/java/de/thm/arsnova/config/AppConfig.java

@@ -104,9 +104,8 @@ import de.thm.arsnova.websocket.ArsnovaSocketioServerImpl;
 @Configuration
 @EnableAsync(mode = AdviceMode.ASPECTJ)
 @EnableAutoConfiguration(exclude = {
-	DataSourceAutoConfiguration.class,
-	DataSourceTransactionManagerAutoConfiguration.class
-})
+		DataSourceAutoConfiguration.class,
+		DataSourceTransactionManagerAutoConfiguration.class})
 @EnableCaching(mode = AdviceMode.ASPECTJ)
 @EnableScheduling
 @EnableSpringConfigured