Dvb Ticket request

[jch4nni]

When registering for attendance at Dev Day, visitors should be offered an input choice for a free ticket for riding the tram to the fairground.

Total number of selection could be displayed in backend to easen the ordering process at DVB.

[jandd]

partly implemented in #322

this should be a proper database field on the event. The current solution is just a short term workaround.