Can't join meeting after meeting host closes a model

[tomgoddard]

If a model is open in a 2-person meeting, then the non-host leaves the meeting, then the host closes the model, then any attempt to join the meeting fails. The problem is that the meeting host keeps a dead references in the meeting_models dictionary that gets references when anyone tries to join in order to send new model positions.

Closed models are not immediately removed from the meeting_models dictionary so all code that uses those models has to check if they are still valid.

[tomgoddard]

Fixed.

I consolidated all the meeting model state in one class and made sure to check if the model has been closed before using it.