Add entry randomization

[PurkkaKoodari]

As discussed in #18, popular events often become F5 wars. Randomizing the signups that are accepted to the event would eliminate the need to catch a signup at opening time, and would also reduce the usefulness of botting. However, it might also appear unfair to people that prefer the usual "first come, first served" principle. Nevertheless, this is an useful feature to support.

This could also be combined with pre-queuing (#18) such that pre-queued entries are randomized but any free slots left are distributed in order of arrival.

The time of randomization might be:

• After pre-queueing, rewarding pre-queuers
• At a manually set time after signup opening, reducing the need for a F5 war without pre-queuing
• After signup close, for complete "arvontalaulu raikaa"