Allow unassigned ("anyone") tickets

- Requires a migration

todo/views.py

@@ -293,9 +293,7 @@ def reorder_tasks(request):
 def external_add(request):
     """
     Allow users who don't have access to the rest of the ticket system to file a ticket in a specific list.
-    This is useful if, for example, a core web team are in a group that can file todos for each other,
-    but you also want students to be able to post trouble tickets to a list just for the sysadmin. This
-    way we don't have to put all users into a group that gives them access to the whole ticket system.
+    Public tickets are unassigned unless settings.DEFAULT_ASSIGNEE exists.
     """
     if request.POST:
         form = AddExternalItemForm(request.POST)
@@ -304,7 +302,8 @@ def external_add(request):
             item = form.save(commit=False)
             item.list_id = settings.DEFAULT_LIST_ID
             item.created_by = request.user
-            item.assigned_to = User.objects.get(username=settings.DEFAULT_ASSIGNEE)
+            if settings.DEFAULT_ASSIGNEE:
+                item.assigned_to = User.objects.get(username=settings.DEFAULT_ASSIGNEE)
             item.save()
 
             email_subject = render_to_string("todo/email/assigned_subject.txt", {'task': item.title})