feat(api): Implemented session closed and kick event via websocket (#87)

Reviewed-on: #87
Reviewed-by: lq64 <lq@blackhole.local>
Co-authored-by: Janis <janis.e.20@gmx.de>
Co-committed-by: Janis <janis.e.20@gmx.de>

knockoutwhistweb/app/events/LeftEvent.scala

@@ -0,0 +1,9 @@
+package events
+
+import model.users.User
+
+case class LeftEvent(user: User) extends UserEvent(user) {
+
+  override def id: String = "LeftEvent"
+  
+}