add WebSockets with STOMP on top

Adds a STOMP broker to the backend. Implements a standalone feedback with messages and topic channels.

This solves #25.

src/test/java/de/thm/arsnova/websocket/FeedbackCommandHandlerTest.java

@@ -51,7 +51,7 @@ public class FeedbackCommandHandlerTest {
 				ArgumentCaptor.forClass(FeedbackChanged.class);
 
 		verify(messagingTemplate).convertAndSend(topicCaptor.capture(), messageCaptor.capture());
-		assertThat(topicCaptor.getValue()).isEqualTo("/room/" + roomId + "/feedback");
+		assertThat(topicCaptor.getValue()).isEqualTo("/room/" + roomId + "/feedback.stream");
 		assertThat(messageCaptor.getValue()).isEqualTo(feedbackChanged);
 	}
 
@@ -69,7 +69,7 @@ public class FeedbackCommandHandlerTest {
 
 		ArgumentCaptor<String> captor = ArgumentCaptor.forClass(String.class);
 		verify(messagingTemplate).convertAndSend(captor.capture(), any(FeedbackChanged.class));
-		assertThat(captor.getValue()).isEqualTo("/room/" + roomId + "/feedback");
+		assertThat(captor.getValue()).isEqualTo("/room/" + roomId + "/feedback.stream");
 	}
 }