Allow projects to be set as policies

Summary:
  - Renames `PhabricatorPolicyQuery` to `PhabricatorPolicyAwareQuery` (a query which respects policy settings).
  - Introduces `PhabricatorPolicyQuery`, which loads available policies (e.g., "member of project X").
  - Introduces `PhabricatorPolicy`, which describes a policy.
  - Allows projects to be set as policies.
  - Allows Paste policies to be edited.
  - Covers crazy cases where you make projects depend on themselves or each other because you are a dastardly villan.

Test Plan: Set paste and project policies, including crazy policies like A -> B -> A, A -> A, etc.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D3476

src/applications/paste/controller/PhabricatorPasteViewController.php

@@ -147,6 +147,14 @@ final class PhabricatorPasteViewController extends PhabricatorPasteController {
         $this->renderHandlesForPHIDs($child_phids));
     }
 
+    $descriptions = PhabricatorPolicyQuery::renderPolicyDescriptions(
+      $user,
+      $paste);
+
+    $properties->addProperty(
+      pht('Visible To'),
+      $descriptions[PhabricatorPolicyCapability::CAN_VIEW]);
+
     return $properties;
   }