jeff/feature-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. 

How do you enable this feature? For a problem, go to its settings in EdX (within a problem, click on the EDIT button and then click on the SETTINGS button). One of the settings listed should read `Timer Between Attempts`. By default, there are 0 seconds a student must wait between submissions, but you can increase this.

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.