Require a viewer to load handles

Summary:
Unmuck almost all of the we-sort-of-have-viewers-some-of-the-time mess.

There are a few notable cases here:

  - I used Omnipotent users when indexing objects for search. I think this is correct; we do policy filtering when showing results.
  - I cheated in a bad way in the Remarkup object rule, but fixing this requires fixing all the PhabricatorRemarkupEngine callsites (there are 85). I'll do that in the next diff.
  - I cheated in a few random places, like when sending mail about package edits. These aren't a big deal.

Test Plan:
  - Grepped for all PhabricatorObjectHandleData references.
  - Gave them viewers.

Reviewers: vrana

Reviewed By: vrana

CC: aran, edward

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D5151

src/applications/differential/controller/DifferentialRevisionViewController.php

@@ -846,10 +846,12 @@ final class DifferentialRevisionViewController extends DifferentialController {
   private function renderOtherRevisions(array $revisions) {
     assert_instances_of($revisions, 'DifferentialRevision');
 
+    $user = $this->getRequest()->getUser();
+
     $view = id(new DifferentialRevisionListView())
       ->setRevisions($revisions)
-      ->setFields(DifferentialRevisionListView::getDefaultFields())
-      ->setUser($this->getRequest()->getUser())
+      ->setFields(DifferentialRevisionListView::getDefaultFields($user))
+      ->setUser($user)
       ->loadAssets();
 
     $phids = $view->getRequiredHandlePHIDs();