Fix unsubscribing from projects in a gross, hacky way

Summary:
Fixes T5261.

This fix isn't very good. Two better fixes would be:

  # Add some sort of `setRole(SUBSCRIPTIONS)` method to `ObjectQuery`, which gets passed down until it reaches `ProjectQuery`, and `ProjectQuery` knows that it needs to load more data. This feels OK, but is a very general approach and I don't think we have many/any other use cases right now. I //think// this is the right way in the long run, but I'd like to have more use cases in mind before implementing it.
  # Add some sort of `loadAllTheSubscriptionStuffYouNeed()` method to `PhabricatorSubscribableInterface`. This feels OK-ish too, but kind of yuck, and doesn't lend itself to proper batching, and is silly if we do the above instead, which I think we probably will.

For now, just fix the issue without committing to an infrastructure direction. I think (1) is the right way to go eventually, but I want a better second use case before writing it, since I might be crazy.

Test Plan: Unsubscribed from a project.

Reviewers: joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T5261

Differential Revision: https://secure.phabricator.com/D9377

src/applications/subscriptions/controller/PhabricatorSubscriptionsEditController.php

@@ -37,10 +37,24 @@ final class PhabricatorSubscriptionsEditController
       ->withPHIDs(array($phid))
       ->executeOne();
 
+    if (phid_get_type($phid) == PhabricatorProjectPHIDTypeProject::TYPECONST) {
+      // TODO: This is a big hack, but a weak argument for adding some kind
+      // of "load for role" feature to ObjectQuery, and also not a really great
+      // argument for adding some kind of "load extra stuff" feature to
+      // SubscriberInterface. Do this for now and wait for the best way forward
+      // to become more clear?
+
+      $object = id(new PhabricatorProjectQuery())
+        ->setViewer($user)
+        ->withPHIDs(array($phid))
+        ->needWatchers(true)
+        ->executeOne();
+    } else {
       $object = id(new PhabricatorObjectQuery())
         ->setViewer($user)
         ->withPHIDs(array($phid))
         ->executeOne();
+    }
 
     if (!($object instanceof PhabricatorSubscribableInterface)) {
       return $this->buildErrorResponse(