Added feature to act as timer between quiz attempts

@nparlante 

Creating pull request for the "timer between quiz attempts" feature on any problem type.

This allows the author to specify how many seconds they want to lock out a student from submitting another attempt of the same quiz. This setting is within the normal UI panel for problem settings.

Note that a student can still start another attempt of the quiz, but will only be disallowed from submitting again until time has elapsed. This is to prevent students from rapidly submitting quiz attempts, particularly for quizzes that offer hundreds of allowed attempts.