Run the HTTP and WebSocket servers on the same port

Use Cask's built-in websocket implementation instead of a separate java_websocket server.

• jvm/src/main/scala/cs214/webapp/server/web/WebServer.scala:
  Remove the WS_PORT parameter.
• jvm/src/main/scala/cs214/webapp/server/web/WebServerRoutes.scala:
  (getAppInfo): Use HTTP_PORT instead of WS_PORT in wsEndpoint
  (websocket): New endpoint.
• jvm/src/main/scala/cs214/webapp/server/web/WebSocketsCollection.scala:
  Remove java_websocket imports.
  Change from WebSocket to cask.WsChannelActor.
  Rename appId to instanceId where appropriate.
  (WebSocketsCollection): Remove the port parameter. (connect): New function, replacing the previous WebSocketServer instance.
• shared/src/main/scala/cs214/webapp/Common.scala:
  (WS_PORT): Remove.
  (WebSocket): New endpoint.
• shared/src/main/scala/cs214/webapp/Common.scala:
  (Endpoints): Add constants for /app and /ws paths.