Create a way to manage PINs valid for classes

[hzeller]

Subject says it all. For classes, we would like to have a PIN code that lets people in only at a particular time at a particluar weekday (recurrent), possibly also for the upstairs door.

We need to have a way to manage that
a) enter/generate such a pin and the time associations. Possibly using the web interface.
b) consider policies around that: should people just get in, or should we still have a greeter assigned ? Maybe the class facilitator gets a quick message on their preferred messaging system.