Limit size of registration

[jfly]

I don't know if this should be a per event limit, or an overall limit on the size of the competition.

@jbcm627 could you comment on what, if anything, cubingusa does here?

I'm unsure if this needs to be done for Phase 1.

[jbmertens]

CubingUSA allows a limit for either total # competitors, or total competitors + guests. Per-event restrictions are typically enforced manually by organizers.

[jfly]

I didn't even think about guests!

[jbmertens]

Size is currently limited so that no new registrations can be added once limits are hit. Existing registrations can still be changed though (including number of guests).

[jfly]

I guess that's slightly unintuitive behavior, but it seems fine to me.