Add secret branches

[evan10s]

Add a new property of a branch that allows it to be accessed only via a special URL. For instance, we frequently have mentors, sponsors, and even our team members do on-site registration, but this process is slow. Secret branches would allow us to send a special link to these groups of people so that we can collect their information in advance and expedite the check-in process because they can present their QR code just like anyone else.