Re-integrate invite system with more customization

[jeda]

It was removed before Cryb went OSS.

It would be great to re-integrate this so we can configure Cryb to be invite-only, be it for both creating/joining rooms or only creating them.

[jeda]

With f9af736, now specific IDs can be whitelisted for room creation, which was the major problem.

A global whitelist that also includes joining rooms would be considered.