Backbone supports to create sprints which are not existent in the target project. Upon creation it also syncs parameters like sprint start/end date.
Backbone should also actual sync sprints. So every time a sprint is changed (e.g. started, completed, dates changed), it should sync the new information to the target project and adapt it accordingly.
We should also sync closed sprints. This might cause problems when starting a new integration with already closed sprints on the other side, since we cannot move Issues into sprints that are closed using the REST API.