Set a limit on the size of the waiting list

[dunkOnIT]

This issue is mostly for consideration in the registration overhaul - we still need to think through the unintended effects/implications of limiting the waiting list.

Having a limit to the size of the waiting list has two benefits:

• Prevent additional stripe fees on payments that will need to be refunded
• Prevent absurdly large waiting lists on popular competitions

Any attempted registrations once the waiting list is full would be rejected on that basis.

[FinnIckler]

The implementation of the early registration closure should probably the best way to figure out how this could be implemented: https://github.com/thewca/worldcubeassociation.org/pull/7526

[dunkOnIT]

#10772