I often do things "for which there is no ticket".  I don't want to invoice them, just record them and maybe later assign them to some ticket. 

Maybe we need a notion of `default_ticket` for unclassified sessions? But where to put that field?  If I simply inject a field `default_ticket` to User, we have a circular reference when dumping which might increase restore time or even cause deadlocks (if the author of a ticket selects this ticket as its default_ticket).  Or a list of recently used tickets? So it seems that the best solution is a field in the global system.SiteConfig. One default ticket for all users.  Why not. To be meditated.