Add a createuser callback for cauth django plugin

This callback can prevent new users from being created based on specific
criteria. The plugin is defined in settings.py, so it can be any python
code. For example, it can look up in a specific table if this user is
supposed to be allowed to log in at all.

tools/communityauth/sample/django/auth.py

@@ -142,6 +142,18 @@ for you.
 We apologize for the inconvenience.
 """ % (data['e'][0], data['u'][0]), content_type='text/plain')
 
+		if hasattr(settings, 'PGAUTH_CREATEUSER_CALLBACK'):
+			res = getattr(settings, 'PGAUTH_CREATEUSER_CALLBACK')(
+				data['u'][0],
+				data['e'][0],
+				['f'][0],
+				data['l'][0],
+			)
+			# If anything is returned, we'll return that as our result.
+			# If None is returned, it means go ahead and create the user.
+			if res:
+				return res
+
 		user = User(username=data['u'][0],
 					first_name=data['f'][0],
 					last_name=data['l'][0],