f[inish #37068821] selectable 0.4.0 and later incompatible with earlier code

see
http://django-selectable.readthedocs.org/en/version-0.5.2/releases.html.

In general, we can't blithely update our imported modules without
reading the release notes.

core/lookups.py

@@ -5,6 +5,6 @@ from django.contrib.auth.models import User
 
 class OwnerLookup(ModelLookup):
     model = User
-    search_field = 'username__icontains'
+    search_fields = ('username__icontains',)
     
 registry.register(OwnerLookup)