Cool down timers

[jessehartloff]

An option for assignments to have a cool down period after a submission where a new submission will not be graded (or can't be made at all). Options for the cool down period set per assignment:

• baseTime: Base cool down time
• freeSubmissions: Number of submissions that can be made before cool down time increases
• timeIncrease: Amount of time added to the base cool down time per submission after freeSubmissions
• maxCoolDown: The maximum cool down time where no more time will be added

The cool down time for a students on an assignment would be:
coolDownTime = min(maxCoolDown, baseTime + max(0, (numberOfSubmissions - freeSubmission) * timeIncrease))