keeps attributes when changing the tracker

This is not yet fully functional by far as the method relies on safe_attributes which does not know some of the new fields, e.g. responsible
fixes start/due_date calendars
moves copying of an issue into the model
returns empty allowed_status if the issue currently has none
no longer warns against a lazy spec implementation if AnonymousUser is set
harmonizes work_package building across issues/pe
initial implementation of work_package_controller#create