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/changesetview/DifferentialChangesetViewController.php

@@ -179,8 +179,7 @@ class DifferentialChangesetViewController extends DifferentialController {
       ->loadHandles();
     $parser->setHandles($handles);
 
-    $factory = new DifferentialMarkupEngineFactory();
-    $engine = $factory->newDifferentialCommentMarkupEngine();
+    $engine = PhabricatorMarkupEngine::newDifferentialMarkupEngine();
     $parser->setMarkupEngine($engine);
 
     if ($request->isAjax()) {