Change how slots for events are prioritized

Right now the priority for event registrations are only sorted by time of registration (with some exceptions for certain priority groups). This means that the server gets DDoS:d every time an event registration opens, and it also means that it's super stressful to register (if for example 100 slots were taken in the first minute!)

This could perhaps be solved with a ”grace period” for the first 2-4 hours? That is, a period where no regard to time registered is made, and then the slots are randomly assigned.