MDL-11699 - Make build_navigation more helpful by having it optionally build the...

MDL-11699 - Make build_navigation more helpful by having it optionally build the activity and activityinstance links too, if you pass a $cm object. Note that this only does extra DB queries if you do pass a $cm object, and it does not have all the required fields, but in this case it will winge at you in developer debug mode. If you get your $cm using get_coursemodule_from_instance or get_coursemodule_from_id, which is best practice these days, then you won't have a problem.