Add a document preview to Phriction

Summary: Preview Phriction documents as they are edited, similar to how
Differential/Maniphest work.
Test Plan: Mashed my keyboard while editing a Phriction document.
Reviewed By: hsb
Reviewers: hsb, codeblock, jungejason, tuomaspelkonen, aran
CC: aran, hsb
Differential Revision: 684

src/aphront/default/configuration/AphrontDefaultApplicationConfiguration.php

@@ -355,6 +355,8 @@ class AphrontDefaultApplicationConfiguration
         'history/(?P<slug>.+/)$'  => 'PhrictionHistoryController',
 
         'edit/(?:(?P<id>\d+)/)?$' => 'PhrictionEditController',
+
+        'preview/$' => 'PhrictionDocumentPreviewController',
       ),
     );
   }