Generalize the markup engine factory

Summary:
This thing services every app but it lives inside Differential right now. Pull
it out, and separate the factory interfaces per-application.

This will let us accommodate changes we need to make for Phriction to support
wiki linking.

Test Plan: Tested remarkup in differential, diffusion, maniphest, people,
slowvote.
Reviewed By: hsb
Reviewers: hsb, codeblock, jungejason, tuomaspelkonen, aran
CC: aran, hsb
Differential Revision: 646

src/applications/differential/controller/commentpreview/DifferentialCommentPreviewController.php

@@ -33,8 +33,7 @@ class DifferentialCommentPreviewController extends DifferentialController {
     $handles = id(new PhabricatorObjectHandleData(array($author_phid)))
       ->loadHandles();
 
-    $factory = new DifferentialMarkupEngineFactory();
-    $engine = $factory->newDifferentialCommentMarkupEngine();
+    $engine = PhabricatorMarkupEngine::newDifferentialMarkupEngine();
 
     $comment = new DifferentialComment();
     $comment->setContent($request->getStr('content'));