Add a filter for the current Sprint in the CFD

When viewing the Cumulative Flow Diagram for a sprint it appears to show all issues where sprint is not EMPTY, i.e. the issue is currently in, or has been in a sprint. It would be nice if the Refine... option had a button to show where sprint in openSprints(), i.e. just the current sprint.

The rationale for this is that the Plan view shows all issues that match the board filter. The Work view shows all issues that match the board filter and are in the current sprint. The CFD chart shows a data set that neither of these views display.