Develop velocity and burndown report improvements

Improve the way the velocity report within Develop handles records that are re-estimated, added or removed during an active sprint.

Specifically would like to see:
- Ability to re-estimate a record after the sprint has started without any adverse impact on the Burndown chart or Velocity report.
- Ability to add or remove a record after the sprint has started without any adverse impact on the Burndown chart or Velocity report.