Make at-event signups configurable per-attraction

[EliAndrewC]

We currently allow people to sign up for attractions before the event, and then turn that off during the event.

The reason was that Autographs wanted to make sure that only checked-in people could sign up for a specific time in the autographs line.

We should allow each attraction to decide whether they want to allow this kind of in-advance signup.

Additionally, we should probably just allow signups without a badge number always. Instead of turning that off, we can instead require that the person signing up be checked in.