Add rate restriction for challenges.

Greg benning suggested that making some of the challenges rate limited might be a good way to ease people into them. This has also been a characteristic of some ctc challenges in the past.

Add an option to include a rate limit to the challenge form. If a rate limit is specified require a integer rate to be entered. Add a step to test average spm (rounded to integer) over the challenge time as less than or equal to the cap.