When computing the "Subscribers" policy, use materialized membership

Summary:
Fixes T13104. The "Subscribers" policy implementation still uses older logic to query project membership and misses parent projects and milestones which a user is a member of.

Instead of doing an edge query for explicit membership, use a project query to find all projects the viewer belongs to.

Test Plan:
  - Created a parent project A.
  - Created a subproject B.
  - As Bailey, created a task with "Visible To: Bailey, Subscribers".
  - Added parent project A as a task subscriber.

Then:

  - As Alice, verified I could not see the task.
  - As Alice, joined subproject B.
    - Before patch: still unable to see the task.
    - After patch: can see the task.
  - Removed parent project A as a subscriber, verified I could no longer see the task.

Maniphest Tasks: T13104

Differential Revision: https://secure.phabricator.com/D19213

src/applications/subscriptions/policyrule/PhabricatorSubscriptionsSubscribersPolicyRule.php

@@ -47,10 +47,14 @@ final class PhabricatorSubscriptionsSubscribersPolicyRule
 
     // Load the project PHIDs the user is a member of.
     if (!isset($this->sourcePHIDs[$viewer_phid])) {
-      $source_phids = PhabricatorEdgeQuery::loadDestinationPHIDs(
-        $viewer_phid,
-        PhabricatorProjectMemberOfProjectEdgeType::EDGECONST);
+      $projects = id(new PhabricatorProjectQuery())
+        ->setViewer($viewer)
+        ->withMemberPHIDs(array($viewer_phid))
+        ->execute();
+
+      $source_phids = mpull($projects, 'getPHID');
       $source_phids[] = $viewer_phid;
+
       $this->sourcePHIDs[$viewer_phid] = $source_phids;
     }