Provide an attached-to-visible-object policy exception for files

Summary:
Ref T603. This uses the existing edges (from Conpherence) to record that a file is attached to an object, and uses those edges to create a policy exception: if you can view an attached object, you can view a file.

I'm going to combine this with restrictive defaults to satisfy the other half of the equation (that files you attach to a conpherence usually shouldn't be public by default).

Test Plan:
  - Loaded `/files/`.
  - Uploaded a file to a Conpherence, looked at it in Files, saw the attachment.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7182

src/applications/files/controller/PhabricatorFileInfoController.php

@@ -27,7 +27,11 @@ final class PhabricatorFileInfoController extends PhabricatorFileController {
       ->withObjectPHIDs(array($phid))
       ->execute();
 
-    $this->loadHandles(array($file->getAuthorPHID()));
+    $handle_phids = array_merge(
+      array($file->getAuthorPHID()),
+      $file->getObjectPHIDs());
+
+    $this->loadHandles($handle_phids);
     $header = id(new PHUIHeaderView())
       ->setHeader($file->getName());
 
@@ -207,6 +211,15 @@ final class PhabricatorFileInfoController extends PhabricatorFileController {
       }
     }
 
+    $phids = $file->getObjectPHIDs();
+    if ($phids) {
+      $view->addSectionHeader(pht('Attached'));
+      $view->addProperty(
+        pht('Attached To'),
+        $this->renderHandlesForPHIDs($phids));
+    }
+
+
     if ($file->isViewableImage()) {
 
       $image = phutil_tag(