Display ongoing synchronization in the header
We should let the user know that the synchronization of their offline events is in progress, by displaying such information in the header of the application. It should replace the information about the "Pending offline events". We don't need to display the progress. We just need to show that it's currently being synchronized.
This should always display when the synchronization is in progress. If there are any synchronization errors and the synchronization is still in progress, we show both information.
We should also display the current progress: Synchronizing... (7 events remaining).