Add pre-queuing to events

[PurkkaKoodari]

Registration to popular events often becomes a F5 (refresh) war, which is effectively randomized participant selection where users with a computer and good internet have an advantage. It also provides an incentive for botting and scalping (even if these haven't been practical issues in current deployments).

Providing a queue system should help with this. The basic idea would be that signups can be created some time before the signup actually opens, but can't be filled in. Ideally this would be implemented such that the tab must be kept open to keep the entry in the queue, and the signup would still need to be filled in a set period after the signups properly open. This ought to provide a smoother experience for everyone.

This can also be combined with entry randomization (#19).