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