Add flag to prevent automatic regeneration of burndown chart

[cornelius]

To prevent generation of the burndown chart with incomplete or inconsistent data we could add a flag to the burndown configuration, which prevents regeneration of the burndown chart. This could be something like sprint_state = done. This could then be used to freeze the burndown chart after final adjustments to the cards have been made, so that cards can freely be manipulated in preparation of the next sprint without automatic burndown generation getting in the way.