Use modern two-stage markup cache (PhabricatorMarkupInterface) in Differential

Summary:
See T1963 for discussion of the Facebook-specific hack.

Differential currently uses a one-stage cache (render -> postprocess -> save in cache) rather than the two-stage cache (render -> save in cache -> postprocess) offered by `PhabricatorMarkupInteface`. This breaks Differential comments coming out of cache for the lightbox, and makes various other things suboptimal (status of handles like @mentions and embeds are not displayed accurately).

Instead, use the modern stuff.

Test Plan:
  - Created preview comments and inlines in Differential.
  - Edited a Differential inline.
  - Submitted main and inline Differential comments.
  - Viewed and edited Differential summary and test plan.
  - Created preview comments and inlines in Diffusion.
  - Submitted comments and inlines in Diffusion.
  - Verified Differential now loads and saves to the generalized markup cache (Diffusion is close, but main comments still hold a single-stage cache).
  - Verified old Differential comments work correctly with the lightbox.

Reviewers: vrana, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1963

Differential Revision: https://secure.phabricator.com/D3804

src/infrastructure/diff/PhabricatorInlineCommentController.php

@@ -222,7 +222,12 @@ abstract class PhabricatorInlineCommentController
     $request = $this->getRequest();
     $user = $request->getUser();
 
-    $engine = PhabricatorMarkupEngine::newDifferentialMarkupEngine();
+    $engine = new PhabricatorMarkupEngine();
+    $engine->setViewer($user);
+    $engine->addObject(
+      $inline,
+      PhabricatorInlineCommentInterface::MARKUP_FIELD_BODY);
+    $engine->process();
 
     $phids = array($user->getPHID());