Check that there are enough competitors to start the next round before opening a round

[CarterKoala]

There was recently a case where WCA Live allowed a 4th round to be opened despite the first round only having 90 people. This led to some results needing to be removed after the competition as it was not realized at the competition that this 4th round was not legal (9m1). A check for this would ensure that this situation would not happen again.