WIP Challenge "Seats"

[rootelement]

We need to add the ability to limit the number of people who can register for a challenge.

Add challenge.seats - If it doesn't exist, or it's set to 0, it's unlimited.

On the challenge registration, when a user registers, make sure there's a seat available.

Questions

1. What prevents someone from registering for a challenge, taking a seat, and not doing the work?
2. Would we use the for public challenges? Or just private/group challenges?
3. Should we add registration requirements somehow? Base rating?