WCRP - Allow competitors to drop events past the registration edit deadline

[Samuel-Baird]

6.4 The registration requirements must include the deadline for updating registered events for
competitors with completed registrations.
6.4.2 Competitors may choose not to compete in an event they are registered for.

As discussed in this issue, the purpose of disallowing competitors to change events to me seems like it is actually to avoid competitors from adding events to their registration. I believe that modifying 6.4 to require a deadline for adding additional events would be beneficial as removing events doesn't cause organizers almost any work (other than updating the registration) and can save organizers the trouble of creating group/staff assignments with incorrect information, printing/cutting unnecessary scorecards, and having to drop "no-show" competitors while score-taking on WCA live.

For the less common competitions that charge by event, rather than a flat fee, I believe that they already have to use external websites for payments, so they should be able to include their own caveats like not allowing changes to competitors registered events past deadlines (as it may cause issues with refunds).

[zekemackay06]

Fixed in #384