Support admins in selecting registrants

[steeevin88]

right now SCEvents ONLY supports a first come first serve registration flow; the first person to fill out the event's registration form has secured a position in the event

that said... we will DEFINITELY have times where admins must be more selective, for instance:

1. hackathons --> ex. we might not want to let certain members in...
2. selective events --> maybe some registrants shouldn't qualify...

at the moment SCEvents already supports unlimited registrations (which is part 0 of this task) --> the next steps should be something like:

• allow setting events to either be first come first server (how they are right now, it should be default) OR this "selective" application
• create the experience for registering for this "selective" application