Add DifferentialDiffQuery and change most callsites

Summary:
Ref T603. This introduces a policy-aware DifferentialDiffQuery and converts most callsites.

I've left unusual callsites (mostly: hard to get the viewer, unusual query, queries related to active diffs) alone for now, so this isn't exhaustive but hits 60-80% of sites.

Test Plan: Created diff; created revision; viewed diffs and revisions; made additional conduit calls.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D6338

src/applications/differential/controller/DifferentialDiffViewController.php

@@ -12,7 +12,10 @@ final class DifferentialDiffViewController extends DifferentialController {
     $request = $this->getRequest();
     $viewer = $request->getUser();
 
-    $diff = id(new DifferentialDiff())->load($this->id);
+    $diff = id(new DifferentialDiffQuery())
+      ->setViewer($viewer)
+      ->withIDs(array($this->id))
+      ->executeOne();
     if (!$diff) {
       return new Aphront404Response();
     }