Use (only) version start / end dates for chartboard "guideline" end points

Currently the chartboard "guideline" end points default to the version start & end dates but the end points change (and guideline redrawn) if the start / end dates on the chart are varied.

While a sprint should be timeboxed there are times when it is useful to show that the scheduled sprint end has not been met and the sprint activity continues beyond this scheduled end date.

If the guideline start / end dates were fixed to the version start / end dates then this would permit showing the activity beyond the scheduled date rather than appearing to show the activity always being on time as the dates are changed on the chartboard