Implement the reset procedure.

This is a lot like the update procedure except:
  * It removes all existing rows first
  * It will calculate the calendar if now is after the end date
Change Update proc to update first day of sprint.

Previously, The update would do nothing if now is before the sprint start
date.  However, it would be convenient to keep the first day up to date
in cases where the sprint is ready to go, but has not officially started
yet.
Merge branch 'feature/SprintCalendarReset' into develop