Don’t allow indexing on service model

Making people use a property is a sure way to make sure they’re spelling
the name of the property correctly, and allows us to easily swap out
properties that call through to the underlying JSON, and properties
which are implemented as methods.

app/notify_client/models.py

@@ -273,6 +273,7 @@ class Service():
 
     ALLOWED_PROPERTIES = {
         'active',
+        'contact_link',
         'dvla_organisation',
         'email_branding',
         'email_from',
@@ -305,7 +306,9 @@ class Service():
         raise AttributeError('`{}` is not a service attribute'.format(attr))
 
     def __getitem__(self, attr):
-        return self.__getattr__(attr)
+        raise NotImplementedError(
+            'Use current_service.{} instead of current_service[\'{}\']'.format(attr, attr)
+        )
 
     def get(self, attr, default=None):
         try: