Feature: close conference registration after a certain number of registrations

[wak-sudo]

Currently, the administrator has to monitor the number of users before closing the registration. It can be inconvenient if the number grows slowly to the limit.

Proposal: add registration auto closing upon reaching a certain number of users with preferences set.

It might be also useful to make this (or possibly another) feature stronger: have registration open iff number of users with preferences set is less than x. This way, the registration could auto toggle if there were free slots left. Of course, this would make more sense if the user had the ability to opt out.