When a batch creates a session, we need to alert the client of this. Because the sessions are created by the batch manager (not directly in response to a client action), we need to emit an event with the session id.
To accomodate this, a secondary set of sio sub/unsub/event handlers are created. These are specifically for batch events. The room is the `batch_id`.
When creating a batch, the client subscribes to this batch room.
When the batch manager creates a batch session, a `batch_session_created` event is emitted in the appropriate room. It includes the session id. The client then may subscribe to the session room, and all socket stuff proceeds as it did before.
