Resolve "there should be a way to add an appointment with external person (neither a subject nor control)"

Closes #28 (closed)

smash/web/models/appointment.py

@@ -47,6 +47,8 @@ class Appointment(models.Model):
     visit = models.ForeignKey(Visit,
                               verbose_name='Visit ID',
                               editable=False,
+                              null=True,
+                              blank=True,
                               )
     comment = models.TextField(max_length=1024,
                                verbose_name='Comment',
@@ -103,7 +105,7 @@ class Appointment(models.Model):
         return result
 
     def title(self):
-        if self.visit.subject.screening_number == "---":
+        if self.visit is None:
             return self.comment.replace("\n", ";").replace("\r", ";")
         else:
             title = self.visit.subject.first_name + " " + self.visit.subject.last_name + " type: "