Make inline comment preview work in Diffusion

Summary: created a PhabricatorInlineCommentPreviewController so controllers in Diffusion and Differential respectively just have to handle the URI mapping and data loading like good little controllers.

Test Plan:
left inline comments on commits, deleted inline commits, submitted inline comments -- all worked well
did the same on some diffs

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1176

Differential Revision: https://secure.phabricator.com/D3034

src/applications/diffusion/controller/DiffusionDiffController.php

@@ -65,6 +65,7 @@ final class DiffusionDiffController extends DiffusionController {
     }
 
     $parser = new DifferentialChangesetParser();
+    $parser->setUser($user);
     $parser->setChangeset($changeset);
     $parser->setRenderingReference($diff_query->getRenderingReference());
     $parser->setMarkupEngine(