Add 'viewer' to some Remarkup callsites

Summary:
I want to implement a `{P123}` rule to embed pastes, but we need viewers everywhere before it will work with privacy.

This is not exhaustive; many Remarkup callsites haven't been converted to `PhabricatorMarkupInterface` yet.

Test Plan: Looked at Maniphest, Differential, Diffusion, Phriction; added markup, made edits and hit previews.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3428

src/applications/maniphest/controller/ManiphestTransactionPreviewController.php

@@ -119,6 +119,7 @@ final class ManiphestTransactionPreviewController extends ManiphestController {
     $transactions[] = $transaction;
 
     $engine = new PhabricatorMarkupEngine();
+    $engine->setViewer($user);
     $engine->addObject($transaction, ManiphestTransaction::MARKUP_FIELD_BODY);
     $engine->process();