I realized that we should maybe start using "tasks" in addition to tickets. A ticket is something that has been introduced by a customer. Until now we said that every ticket is a task in the todo list of the worker who is assigned. Sometimes a single ticket can lead to different tasks. So we should have a TasksByTicket table. Tasks are currently part of the xl.lib.cal plugin, but we should problably split them into their own plugin. Note that this would also change what I did for #1810 (cal.Task should also become Assignable)