allow "burst submissions" #1683

This is something that Kattis apparently has. Basically relax the submission rate limit from "1 minute since last submission" to "either 1 minute from last submission OR there have been less than 15 submissions in the past 15 minutes".

I think it could be configurable via a new "burst size" parameter, so together with the current "min interval" parameter the rule is "min_interval seconds since last submission OR less than burst_size submissions in past min_interval * burst_size seconds".