Don't require missing fields be present in order to subscribe to an object

Summary: Fixes T8719. It's fine to subscribe to objects that don't have all their required fields filled in.

Test Plan: Subscribed/unsubscribed from an object.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8719

Differential Revision: https://secure.phabricator.com/D13484

src/applications/subscriptions/controller/PhabricatorSubscriptionsEditController.php

@@ -95,6 +95,7 @@ final class PhabricatorSubscriptionsEditController
       $editor = id($object->getApplicationTransactionEditor())
         ->setActor($user)
         ->setContinueOnNoEffect(true)
+        ->setContinueOnMissingFields(true)
         ->setContentSourceFromRequest($request);
 
       $editor->applyTransactions(