Feature Request: Generate and require user join codes

Allow admins to generate a list of single-use join codes that are recorded in the database and then required (and consumed) upon user registration. These codes can then be printed and handed out at in-person conference events or shared electronically via out-of-band invitation mechanisms allowing admins to have better control over registration.

This would help with running CTFs that are hosted on the internet but aren't intended to be open-registration, and could also inhibit the usage of additional user/team registration as a way to bypass challenge attempt limits.