implement functionality which make chat messages persistent

build.gradle

...tlin/de/thk/gm/websocketsdemo/configs/WebSocketsConfig.kt

.../thk/gm/websocketsdemo/controllers/ChatRoomsController.kt

...lin/de/thk/gm/websocketsdemo/handlers/ChatRoomsHandler.kt

...ain/kotlin/de/thk/gm/websocketsdemo/models/ChatMessage.kt

...k/gm/websocketsdemo/repositories/ChatMessageRepository.kt

...n/de/thk/gm/websocketsdemo/services/ChatMessageService.kt

.../thk/gm/websocketsdemo/services/ChatMessageServiceImpl.kt

src/main/resources/templates/chatRooms/showChatRoom.ftlh