Publish AnswersChanged events to topic for WebSocket

The event is published after bulk creation and deletion of answers. The full AnswerStatistics for the affected content are sent with the AnswersChanged WS event in addition to the IDs of changed answers.

A new BulkChangeEvent (internal event system) is published after multiple entities have been patched or deleted. It can (and should) also be used after bulk creation but there currently is no generalized bulk creation method for entities, so publishing needs to be handled explicitly by the respective entity service. This is the case for the AnswerService's scheduled bulk creation.